In Roscommon they were long associated with the royal O'Connors, traditionally deriving from the same stock, and supplying stewards to the royal household. The centre of their power was between Mantua and Elphin, near the modern town of Castlerea.
In Fermanagh the Flanagans were part of the Cenel Cairbre, the group of families claiming common descent from Cairbre, one of the sons of Niall of the Nine Hostages, the founder of the Ui Neill dynasty. They were rulers of a large territory covering the west of Lower Lough Erne, and based at Ballyflanagan, now the townland of Aghamore in Magheraboy parish. Today the surname is found widely distributed throughout Ireland, though the largest concentration remains in the areas of their original homelands, south-west Ulster and north Connacht.
In Monaghan the family descend from Flannacan, son of Fogartach King of Farney, whose death is reported in 886. They appear to have been based in the parish of Donaghmoyne.
The Offaly family lived in the barony of Ballybritt, in the south of the county.
The arms are of the Connacht family, and display the royal oak, symbol of the O'Connors, proclaiming their long association with this family, though the tree does not cover the whole shield, a significant difference.
Roderick Flanagan (1828-1861) emigrated to Australia, where he founded the Sydney Chronicle.
John J. Flanagan ( 1873-1938), born in Co. Limerick, won three gold medals for hammer-throwing in 1900, 1904 and 1908..
T.P Flanagan (1929 - ) is one of the best known Irish contemporary landscape artists.
Father Michael O?Flanagan (1876-1942), of the Roscommon family, was prominent in Republican politics, becoming vice-chairman of Sinn Fein and vice-president of the Gaelic League.
Kevin O?Flanagan (1919 - ) was one of the country?s most vesatile sportsmen. He was Irish 100 yards champion and long jump champion, as well as playing both rugby and soccer for Ireland internationally.