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This surname is practically the same in both its Irish and Anglicized forms, being in the former O Flannagain, which is probably derived from the adjective flann meaning reddish or ruddy. It belongs to Connacht both by origin and location (I.e present distribution of population). Flanagan, with of course O'Flanagan, for this is one of those names with which the prefix is frequently retained, is numbered among the hundred commonest surnames in Ireland and has the sixth-ninth place on that list. The greatest number of these are found in Co.. Roscommon and in the counties of the western seaboard - Mayo, Galway and Clare. They sprang from one Flanagan, who was of the same stock as the royal O'Connors and his line held the hereditary post of steward to the Kings of Connacht. These, who were seated between Mantua and Elphin, represent the main O'Flanagan sept. There were also minor septs of the same name in other parts of the country which were still represented in the seventeenth century; of Toorah in north-west Fermanagh and again of the barony of Ballybrit in Offaly. Some descendants of these are still to be found in both these areas.
Donough O'Flanagan (d. 1308), Bishop of Elphin, was famous abroad as well as at home for his hospitality and devotion. Other notable Irishmen of the name were Roderick Flanagan (1828-1861), founder of the Sydney Chronicle: Thomas Flanagan (1814-1865), author of the History of the Church in England; and James Roderick Flanagan (1814-1900), voluminous author on Irish subjects. Theophilus O'Flanagan (1760-1818), was a leading figure in the early Gaelic revival movement.