genealogies of the families
(O)KEENAN The sept of O'Keenan - ÓCianáin in Irish - is chiefly remarkable for the number of distinguished ecclesiastics and historians it produced in the middle ages. Between 1345 and 1508 no less than eight are mentioned by the Four Masters as historians to the Maguires of Fermanagh. The most famous of these was Adam ÓCianáin, Canon of Lisgool, while one Rory O'Keenan was chief scribe of the "Book of Magauran". The Abbey of Lisgool is in Co. Fermanagh and this, running through Co. Monaghan into Louth, and comprising the territory formerly known as Oriel, is the home country of the Keenans nowadays as in mediaeval times. The name Keenan is not to be confused with Keenahan which is quite distinct from it. The best known Keenans are Frank Keenan (1858-1929), American actor, son of an Irish emigrant, Sir Norbert Keenan (b. Dublin 1866), Australian statesman, Sir Patrick Keenan (1829-1894), Chief Commissioner of National Education in Ireland and Joseph Henry Keenan (b. 1900) the authority of thermodynamics. The form MacKeenan appears in the census of 1659 as numerous in Leitrim and elsewhere, but the prefix Mac is now quite obsolete and may be due to a mistake on the part of the seventeenth century non-Irish officials who were often confused between O and Mac. It is always O in the native Irish Annals.
Source: Irish Families by Edward MacLysaght MA, D Litt, MRIA - Irish Academic Press 1991