Erin comes from a gaelic goddess... Eire or Eiry or some such thing... would prolly have more luck finding it in celtic myths and legends... I doubt there is a feast day associated with it as the catholics and other christians wouldn't have celebrated her... people in
Ireland wouldn't have named their children
Erin, it would have been more common for ex-patriots or Americans with Irish roots to give this name...
Scye