Why not start by adding some decorations to the entrance to your special Irish Pub by hanging up a sign, laying down an Irish welcome mat and adding a green light to the entrance.
As your guests arrive, why not give them names that match their personalities. You could even make them special name tags if you would like. Names like:
"P. Ann O'Lesson"
Make sure to have plenty of hats, necklaces, streamers, confetti balloons, shamrocks, and yes, even some green feather boas.
Place all the articles on your serving tables as table decorations, but encourage your guests to pick up what they want to wear.
Find some great Irish music and make sure you have it playing in the background. There are many good Irish bands and some even play the traditional songs with a bit of a Rock influence.
Encourage your guests to dress for the part with green eye shadow, green hair, green nail polish and maybe some green shamrock temporary tattoos.
Decorate with posters of Ireland and Guinness Beer. Print out and hang up some of the the old Irish toasts and blessings.