This is an Anglicized form of three distinct Gaelic (Irish) surnames. The first, "O'Seanain" with the cognate "MacGiolla tSeanain", translates as "descendant of the follower of St. Senan". St. Senan was a 6th Century Irish abbot-bishop who settled on Scattery Island in the Shannon Estuary. These names normally Anglicized as (Mac)Giltenan in Counties Wexford and Carlow, became Shannon in Co. Clare. A second West Clare name "O'Sionain", originally given to a worker with straw (i.e. a maker of straw baskets, straw festival garments, etc.), is rendered Shinane or Shannon in English. Finally, "O'Seannachain", a Dalcassian (Munster) surname is Anglicized both as Shannahan and Shannon. The Gaelic prefix "O" indicates "male descendant of", plus the personal name "Seannachain" from "Seain", old and senior. Prior to the 14th Century, the chief of the last mentioned clan held territory between Bodyke and Feakle in Co. Clare. The first recorded spelling of the family name is shown to be that of O' Seannachain, a chief of the name, which was dated 1318, in the "History of Delcassian (Co. Clare) Clans", during the reign of King Edward 11 of England, known as "Edward of Caernafon", 1307 - Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.1327.

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