The best and easiest choice is probably a t-shirt. I've had t-shirts made in the past, and if I can get my act together in time this year I'll do the same. Still, I always like to see if there are any new alternatives to the Kiss-Me-I'm Irish standby—they make good gifts.
I found The Irish T-shirt Shop this morning and I'm liking their shirts featuring popular Irish phrases and slang. Here are a few of my favorites (all about $18.50 and available in green or white):
Love this Eejit shirt—"Eejit" means idiot (you can also buy a "Feckin Eejit" shirt). An Irishman walks into the bar" references the first line of every Irish joke. Besides, where else would he be going?"Suckin' Diesel" is a common slang for "doing well," i.e., "How'r'ye now?" "Ah sure I'm suckin' diesel." "Póg mo thóin" is Irish for "kiss my arse!"
"How's she cuttin?" means "how's it going?" "Nuns! Nuns! Reverse! Reverse!" is a classic line, taken from hugely popular and hilarious Irish TV show, Father Ted. You don't have to be a Father-Ted fan to find it funny, though!