Shannon, It is difficult to be certain as to which Gaelic surname this is an anglicisation. ó Seanáin derived from a diminutive of sean meaning ‘old, wise’. Also a Scottish surname of similar derivation, found in Galloway and Kintyre. See ó Seanacháin (also anglicised as Shanahan). In 1890 the surname was principally found in Antrim, Clare and Roscommon, and the estimated number of bearers was 3,225. In the United States Shannon is the 679th most numerous surname with an estimated 46,750 bearers.