The Fitzpatrick Y-DNA study has found there are several large and distinct groups of Fitzpatricks.
Aligning the results of the DNA study with historical records and genealogies has enabled some of these larger genetic groups to also be defined by geographic location.
There are the ancient Mac Giolla Phádraig Dál gCais (Fitzpatrick of the tribe of Cas) who are of haplotype R-Z253 ... L226, who descend from Scanlan Mac Giolla Phádraig of Ossory, the chieftain of various annalistic records and pedigrees. Also ancient are the Mac Giolla Phádraig of Ulaidh (Fitzpatrick of Ulster), who are R-FGC11134 ... BY12234, and the Mac Giolla Phádraig Laighin (Fitzpatrick of Leinster), who are R-Z255 ... BY2849. All three clans are registered with the Clans of Ireland.
Notable because they likely bore a surname now extinct, are the Ó Maol Phádraig Bréifne (O'Mulpatrick of Breifne), who are R-FGC5494 ... FT375991, and who are also registered with the Clans of Ireland. It is probable the numerous Fitzpatricks who are under haplotype R-FGC11134 ... CTS4466 are Ó Maol Phádraig of Mhumhain (O'Mulpatrick of Munster), who appear to be a branch of the O'Sullivan that emerged in the fourteenth century.
Also later emerging (ca. 1450 AD), and from a Norman-Irish line, are the Fitzpatrick of Counties Tipperary, Kilkenny and Laois (haplotype R-FGC5494...A1488). Among this haplotype are those who descend from the Mac Giolla Phádraig clerics of Ossory and, probably, the Barons of Upper Ossory.
In addition to the larger groups mentioned, there are many smaller genetic groups that include haplotypes R-L513, R-M222, R-U106, I-M223 and J-M172. The L513 group is particularly interesting because it is strongly associated with the surname Maguire and, therefore, may represent descendants of Gilla Phádraig of Fermanagh of the Maguires.
And then there are many individuals with no close Y-DNA matches to any other Fitzpatricks; they include men identified by haplotypes R-Z253, R-CTS1751, D, E, I and Q-M242.