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(O)Shannon, O Shanahan, Gilshenan, Giltenan
The Gaelic names of three distinct Irish families became anglicized as Shannon or O'Shannon. First there is O Seanain (descendant of Senan, a personal name) of which we know little beyond the fact that it was associated with Counties Carlow and Wexford, where, however, the name is now rare. Another derivation from the same personal name is Mac Giolla t-Seanain (son of the follower of St. Seanan), which, though normally anglicized Giltenan, has become Shannon in Co. Clare. The cognate MacGiolla Seanain anglicized Gilshenan etc., the name of a Tyrone-Fermanagh sept, is not rendered Shannon in recent times; but the census of 1659 (in which prefixes Mac and O were often confused) gives O'Seanan as one of the principal Irish names in the district around Enniskillen, Co. Fermanagh, at that date. A further complication arises insomuch as in Co. Clare, where, with Belfast, the name Shannon is found most plentifully, it is a synonym of Shanahan (in Irish O Seanachain). This is a case of contraction due to English influence: English surveyors and law clerks (being like many English of the present day inclined to silence an internal H) at the time of the Act of Settlement and again after the Williamite confiscations wrote down Irish names as nearly as they could phonetically, hence Shanahan was recorded Shannon. The O'Shannahans were a Dalcassian sept of sufficient importance to have a recognized chief in early times: of the clan Ui Bloid, his territory lay between Bodyke and Feakle in Co. Clare where the name still survives; but in the year 1318 he and his followers were dispossessed by the MacNamaras and in the fourteenth century they became dispersed all over Munster. Curiously enough, though one would expect less foreign influence in the west, the form Shannon Is normal in Clare, but Shanahan is commoner in the other Munster counties. The total number of births recorded is approximately the same for Shannon and Shanahan (I.e. sixty per year each).
It should be noted that the surname Shannon is unconnected with the name of the principal river in Ireland.
The name does not appear frequently in the history of the country or among its notabilities in the cultural sphere. The well known London portrait painter Charles Shannon (1863-1937), was presumably of Irish extraction. A prominent Labour leader and writer in our own day was Cathal O'Shannon and, in America, Wilson Shannon (1802-1877), the governor of Ohio, made his mark as lawyer and diplomat.