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O'Meehan in Irish O Miadhachain, is the name of a sept belonging to Co. Leitrim: this sept (also called Meighan) is of the same stock as the MacCarthys of south Munster, but by the end of the eleventh century they had migrated and established themselves in their new country, their association there with being perpetuated by the place name Ballagh-meehin, or Ballymeighan, in the parish of Rossinver, Co. Leitrim. Thence they spread into adjacent counties and are now fairly numerous in all east Connacht, and in Co.. Clare where they are mentioned in 1317 as one of the Thomond septs which rallied to O'Brien; they also appear in the 1659 census as more numerous in Co. Clare than elsewhere.
Thomas and Denis O'Miachan (O'Meehan) were successively bishop of Anchonry between 1251 and 1285, (the authority for this statement was Analecta Hibernica VI; the subsequent published British Handbook of Chronology corrects this, the name of the first being Thomas O'Maicin not O Miachain), and another bishop, Edru O'Meighan, held the see of Meath from 1152, when he attended the Council of Kells, until 1173. Another distinguished ecclesiastic was the historian Rev. Charles Patrick Meehan (1812-1890). Two of the sept distinguished themselves in France: Count James Anthony Mehegan (alias Meehan) (1719-1792), son of Chevalier O'Mehegan, as a soldier and his brother Chevalier William Alexander Mehegan (1721-1766) as a French author.
The most notable fact relating to the sept is of a religious character: a metal case containing a manuscript of St. Molaise of Devenish (sixth century) was for over a thousand years preserved by successive generations of O'Meehans, and is now in the National Museum of Ireland.