Celtic knot Game :
Irish Dancing: doesn't have to be 100% authentic for them to enjoy themselves and tire themselves out at the same time! Tapes/videos of Irish music and / or Riverdance.
Hurling or Gaelic football, but have first aid kit ready.
Irish pop groups.
Irish food : Potatoes, bacon/gammon, Irish Stew, Colcannon, Soda Bread (see recipes on http://traditions-uk.freeservers.com/)
Come to the meeting dressed in green!
Peg dolls - dress them in green materials (design own costume) or dress as leprechauns, or in traditional Irish dancers' costume, or....? (Basic peg doll instructions here.)
Background information on St. Patrick on this page.