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: habitational name, possibly from Cogill in Aysgarth, North Yorkshire, which is named with Old English cot ‘cottage’ + Old Norse kelda ‘spring’, or perhaps from any of the numerous places named Cowgill or Cow Gill (see Cowgill).