(O')Gallagher in Irish is Ó Gallcobhar, from gall, meaning "foreign" and cabhair, meaning "help" or "support". The original Gallcobhar from whom the family claim descent was himself sixth in descent from Conall Gulban, son of Niall of the Nine Hostages, the fifth century monarch who was reputedly responsible for the kidnapping of St. Patrick to Ireland and who was the founder of the Uí Néill dynasty. The O'Gallaghers claim to be the most senior branch of the Cenél Conaill, the group of families who all descend from Conall Gulban. Their territory was in Tír Chonaill (literally "Conall's Land"), in what is now Co. Donegal. Despite the claim to seniority, early references to the family are sparse. From the fourteenth to the sixteenth centuries, however, they were hereditary commanders of the cavalry of the forces of the O'Donnell princes of Tir Chonaill.
Today Gallagher is the single most numerous surname in Co. Donegal, and is also very common in the adjoining counties of Derry, Fermanagh and Tyrone. Though less common elsewhere in Ireland, it has spread throughout the country over the centuries. More than 30 variant spellings of the name have been recorded.
In 1890, the surname was ranked 12th most common in Ireland, with 422 births of the name. In 1996 it was ranked 20th.
Redmond O’Gallagher (1521-1601), one of the very many O’Gallaghers recorded in Irish church history, was an enthusiastic supporter of O’Donnell and the other northern Lords in their struggle with the English and was killed in that struggle.
William Davis Gallagher (1808-1894) was a well-known American poet. His father had fled Ireland after the Emmett rebellion of 1803.
Paddy "the Cope" Gallagher (1873-1964) was a tireless promoter of rural co-operatives, transforming his home town Dungloe in the 1950s and 60s. His nickname came from the local pronunciation of "co-op". His son, Pat "the Cope", is a member of the European Parliament.
Rory Gallagher (1949-1995) was born in Co. Donegal and raised in Cork. He first became well-known as part of the blues trio Taste in the late 1960s, and went on to become a major blues guitarist with a huge international following.