The Rev. Patrick Woulfe (1872-1933), or as he preferred to be known, An tAthair Pádraig de Bhulbh, was born in Cratloe to Seamus Woulfe, a farmer. He became a priest, worked in Limerick, and spent 25 years collecting names, communicating with native Irish speakers and studying the different forms of Gaelic to compile the Irish Names and Surnames dictionary. There are two versions of his book – a small version (138 pages) and a larger version (742 pages). The smaller version was originally published in 1906 and reprinted in 1922 – this is available online in its as original print form from Archive.org here. The larger version was printed in 1923 (reprinted in 1967) and consists of 696 pages, with 46 pages of preliminary text. A digital version of the book is available free of charge online from Library Ireland here. The search feature is very useful and allows you to find any mention of a particular name within the book. Woulfe’s work helped to popularise the use of Irish first-names during the last century and has been an important resource for genealogists. His work was subsequently superseded by that of other scholars, such as Edward MacLysaght.
Woulfe, Patrick. Sloinnte Gaedheal is Gall: Irish Names and Surnames, collected and edited with explanatory and historical notes (1923).
Edward MacLysaght was an interesting character. He lived to be 99 years old (1887-1986) and packed a lot into his lifetime. His father was from Cork, his mother from Lincolnshire, and he was born near Bristol. He went to school in Rugby and then Corpus Christi College, Oxford to study law. But a rugby injury changed his life - he went to Lahinch in Co. Clare and lived in a caravan for 6 months recovering. During this time he developed a love for genealogy, history, and the Irish language, in which he became fluent. He started a pioneer farm in Raheen and introduced electricity 40 years before his neighbours. He also set up local community projects and was deeply involved in the Irish Cultural Revival and the movement for Irish independence. His loyal support of Eamonn de Valera (then member of parliament, and later President of Ireland) resulted in his imprisonment. After independence (1922), he was elected to the Seanad Éireann (the Irish Senate). He later became an Inspector for the Irish Manuscripts Commission (1938), a member of the Royal Irish Academy (1942), Chief Herald of Ireland (1943-1954), Keeper of Manuscripts at the National Library of Ireland (1948-1954), and Chairman of the Irish Manuscripts Commission (1956-1973). It was during this latter period that he wrote his indispensible books on Irish surnames.
MacLysaght, Edward. The Surnames of Ireland. 1957 (sixth edition 1991)This is a comprehensive list of over 4000 Irish surnames with a short description of each. It includes 1500 surnames not considered in the other books below, which only deal with about 20% of the surnames described in this present book (albeit in a lot more detail).
MacLysaght, Edward. Irish Families. Their Names, Arms and Origins. 1957 (fourth edition 1985)
MacLysaght, Edward. More Irish Families. 1970 (first paperback edition 1996, incorporating Supplement to Irish Families, 1964)These latter two volumes (frequently abbreviated to IF and MIF) give a much more detailed account of many of the most common Irish surnames. Second hand copies can be bought online, usually for quite inflated prices.
Woulfe’s Surname Dictionary
Woulfe’s entry for Farrell as a surname can be found here - http://www.libraryireland.com/names/of/o-fearghail.php
Ó FEARGHAIL—I—O Ferrall, O'Farrell, Farrell, Ferrall, Farrahill, Frahill, Fraul; 'descendant of Fearghal' (super-valour); the name of several distinct families, of which the best known are the O'Farrells of Annaly, in the present Co. Longford, of which they were for many centuries the ruling race. The head of the family resided at the town of Longford, which was formerly known as Longphort Ui Fhearghail, or O'Farrell's fortress. In later times, the O'Farrells divided into two great branches, the heads of which were known respectively as O'Farrell Boy, the yellow O'Farrell, and O'Farrell Bane, the fair O'Farrell. The O'Farrells maintained their independence as a clan down to the year 1565, when Annaly was reduced to shire ground by the lord-deputy, Sir Henry Sidney. Though suffering severely from the plantation schemes of James I, the O'Farrells were able to take a prominent part in all the political and military movements of the 17th century, and many of them were afterwards distinguished officers in the Irish brigades in the service of France. This family is now very numerous. Other families of this name were seated in Wicklow and Tyrone. The name is also written Ó Fearghaile and Ó Firghil, ... and sometimes, by the aspiration of the initial f, changed into Ó hEarghail, Ó hEarghaile.
And the entry for Farrell as a forename or personal name reads as follows:
FEARGHAL, genitive -ghail and -ghaile, Fergal, Farrell; an ancient and once very common name, especially among the MacDonnells, MacDonoughs, Mageoghegans, O'Farrells, O'Neills and O'Rourkes; still in use, but rare. It is supposed to have been the Irish name of the celebrated St. Virgilius, the Irish fear- having been equated with the Latin vir-(man). Latin — Fergalius.
MacLysaght’s Surname Dictionary
MacLysaght gives a brief account of the Farrell surname in his Surnames of Ireland and a much more detailed account of the Farrell name in his Irish Families.
(O) Farrell, Ferrall Ó Fearghail (man of valour). Numerous and important sept of Annaly whose chief’s seat was Longford, formerly called Longphort Ui Fhearghail (i.e. O’Farrell’s fortress). The name is now widespread throughout the four provinces. IF Map Longford
(from Surnames of Ireland, p104)
The abbreviation IF refers to his book Irish Families, and Map Longford refers the reader to the map in that book on page 223.
(O)FARRELL, (O)Ferrall Farrell, with and without the prefix O, is a well known name in many parts of the country and it stands thirty-fifth in the statistical returns showing the hundred commonest names in Ireland. It is estimated that there are over thirteen thousand of the name in Ireland; the great majority of those were born in Leinster, mainly in Co. Longford and the surrounding areas. This is as might be expected for the great Ó Fearghaill (O’Farrell or O’Ferrall) sept was of Annaly in Co. Longford. The chief of the sept, known as Lord of Annaly, resided at Longphuirt Ui Fhearghaill (i.e. O’Farrell’s fortress), hence the name of the town and county. So important were they that references to them in the “Annals of the Four Masters” occupy more than seven columns of the index to that monumental work. There were two branches of the sept, the chiefs of which were distinguished as O’Farrell Boy (buidhe, i.e. yellow) and O’Farrell Bane (bán, i.e. white or fair).
There were a number of distinguished churchmen of the name, of whom the Capuchin Father Richard O’Farrell (c. 1615-1663), of Annaly, was perhaps the most notable. Notwithstanding the misfortunes which befell the great Gaelic families through the conquests and confiscations of the sixteenth and seventeenth centuries the O’Farrells of Annaly were not entirely submerged and many of them took a worthy part in Irish resistance to English aggression. Three sons of Ceadagh O’Ferrall of Annaly, who was killed at the Battle of the Boyne in 1691, greatly distinguished themselves as officers of the Irish Brigade in the service of France. The family settled in Picardy. Later on in the political field Richard More O’Ferrall (1797-1880) was a prominent supporter of Daniel O’Connell. Sir Thomas Farrell (1827-1900) was a noted sculptor, many of whose statues adorn the city of Dublin. The compiler of one of the best known Irish genealogical manuscripts, “Linea Antiqua” (1709) now in the Genealogical Office, Dublin, was Roger O’Ferrall.
(from Irish Families p84)Implications for DNA Research
It is interesting that Woulfe says that the name Farrell represents several distinct families. This is not surprising given the meaning of the name and one can easily imagine that many different families may have had a "man of valour" among their stock from whom they subsequently took their family name. However both Woulfe and MacLysaght focus on the Longford Farrells and only Woulfe makes mention of other possible origins, namely Wicklow and Tyrone, where "other families of this name were seated".
If the surname is indeed multi-origin, then one would expect to see several distinct genetic signatures among Farrells alive today. If there are surviving descendants, and enough people test, we might see several Farrell signatures, perhaps localised to the places mentioned in the surname dictionaries - Longford, Wicklow, & Tyrone.
As the Farrell name is of Gaelic origin, and would probably have been introduced between 900-1100 AD, one would expect to see the dominant haplogroup of all Farrell septs to be Haplogroup R1b, particularly R1b-L21. Haplogroup R1b is the most common haplogroup in Western Europe, with highest concentrations in Ireland (moreso as one moves west). The Neolithic people who built Newgrange (3200 BC) were probably Haplogroup G2a and I2a1; the majority of the Proto-Celts who arrived about 2000 BC were Haplogroup R1b-L21, making it the quintessentially Gaelic lineage; Norwegian Vikings (800 AD onwards) would tend to be R1a-Z284 (they settled in Leinster & Munster); whilst the Anglo-Normans (1169 onwards) would be largely a mix of I1, I2a2a, R1a, and R1b-U106. 
Of the genetic families currently identified in the Farrell DNA project, 5 families belong to Haplogroup R1b, consistent with a Celtic origin. A large number of project members remain ungrouped, and this would be consistent with a multi-origin surname.
The characteristics and possible origins of each genetic family will be explored in subsequent posts. At this stage in the project, it appears that certain potential surname variants have their own specific genetic signature - for example, people with the surname Farley are found predominantly in genetic family R1b-GF1, people with the potential surname variant Farrar are found in R1a-GF1 (see also the Farrar DNA Project), and people with the Ferrell variant predominate in R1b-GF2.
As more people join the DNA project, the association (or lack thereof) between the potential surname variants will become more clear. It may also be possible to localise the origins of each genetic family to a particular place and even to a particular era.
Furthermore, both Woulfe and MacLysaght make mention of the two major branches of the clan in Longford, the O'Farrell Boy and the O'Farrell Bane. These two branches apparently spring from the same root and thus would be expected to have similar genetic signatures, but there may be distinct genetic differences that distinguish these two branches. When the split took place is not mentioned by either author but DNA testing (including SNP markers) could help date this divide.
More information about the history of the Farrell surname can be found on the Farrell Clan website and the wikipedia entry for the Farrell clan.
 Genetic History of the British and the Irish, available at http://www.eupedia.com/genetics/britain_ireland_dna.shtml