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The surname Fitzpatrick is the known translation of at least two different surnames Mac Giolla Phádraig and Ó Maol Phádraig from the original Irish to English. While both Mac Giolla Phádraig and Ó Maol Phádraig have similar meanings, they are historically unrelated; yet both have arrived in the modern era as Fitzpatrick. The name is one of only two attested surnames of native Gaelic-Irish origin with the Norman French "Fitz" prefix. All others are of Hiberno-Norman descent.
Giolla Phádraig (meaning "the devotee of Patrick") was a tenth century king of Ossory, a kingdom in between Munster and Leinster in Ireland. According to Carrigan, this kingdom was founded by Aengus Osrithe who flourished some time about the latter half of the 2nd century of the Christian era. Giolla Phádraig's reign commenced some eight centuries later in 976 AD and he reigned until he was slain in 996 AD. His sons were subsequently styled Mac Giolla Phádraig (meaning, son of Giolla Phádraig). After the time of the Norman invasion, some Irish names were given anglicised equivalents; "Fitz-Patrick" was the equivalent for "Mac Giolla Phádraig".
By far the most important branch of the sept is the family whose Chief was known as Lord of Upper Ossory. At one time he was royal ruler over Laois and neighboring Kilkenny. Following the Norman invasion in the late 12th century, their power was vastly diminished by the ascendancy of the Ormond Butlers and other Hiberno-Norman magnates. Although their patrimony was restricted to Upper Ossory, the Fitzpatricks were by no means dispossessed of all their property. They were one of the original great Irish families to submit to Henry VIII, and as a result, in 1541, Brian, the first to assume the surname Fitzpatrick in place of Mac Gìolla Phádraig, was created Baron Upper Ossory in the Irish House of Lords. In the 17th century, the Fitzpatricks lost considerable territory through their staunch support of James II. Nevertheless, the head of the sept received a peerage in 1714 as Baron Gowran which was elevated to Earl of Upper Ossory in 1751. A third title Baron Castletown was granted in 1869. Records from 1878 show that no less than 22,000 acres (89 km²) of the finest land in Ossory was owned by the family.
The clan name is found throughout the world and several individuals and families have been prominent. Apart from the Lords and Earls of Upper Ossory, also prominent was Brian Mac Giolla Phádraig (1585–1652), Vicar Apostolic of Ossory, who was murdered by Cromwellian soldiers. He was instrumental in saving the "Book of the O'Byrne", which he had transcribed, from destruction. The family of Sir Percy Fitzpatrick made a major contribution to the political formation of South Africa. Patrick Fitzpatrick (1792–1865) was a trusted colleague of Daniel O'Connell. Fitzpatricks have also contributed, with significant influence and success, in high office in Ireland, England, Canada, India, Australia and the United States.
There is a concentration of Fitzpatricks in County Cavan, County Laois and the border counties of Ireland, such as , Fermanagh, Monaghan, Armagh, and County Down.
The Fitzpatrick (Mac Giolla Phádraig) motto - Fortis sub Forte Fatiscet - can be interpreted as "The strong will yield to the strong." A second motto, "Ceart ládir abú" translates to "Right and Mighty Forever".
The Fitzpatricks of the border counties of Cavan, Monaghan and Fermanagh are the descendants of the O' Maol Phadraig (O'Mulpatrick) who originated from Clogher in Co Tyrone. Major Thomas (Broken-Hand) Fitzpatrick, from Cavan, Indian Agent to the Sioux, Cheyenne and Arapaho tribes brought peace to the Plains Indians of North America in 1851 at Fort Laramie.
- The Fitzpatrick - Mac Giolla Phádraig Clan Society, http://www.fitzsoc.com/
- The other is FitzDermot (originally Mac Gilla Mo-Cholmóg).
- Annals of Ulster 996.2