From the name of a river in Ireland. It is not clear why it has become so popular as a given name; compare Clodagh. In part it may also be a transferred use of the Irish surname, Gaelic Ó Seanáin ‘descendant of Seanán’ (a diminutive of Seán). Shannon is not found as a traditional given name in Ireland itself.
English: 1. topographic name for someone who lived by a postern gate, from Old French posterne; in some cases it would have been a metonymic occupational name for a gatekeeper. 2. habitational name from Poston in Herefordshire or Poston in Shropshire, which is named with an Old English personal name Possa + þorn ‘thorn tree’.