what are the must-buy (non-Alcohol) things from Ireland?
From spending a year or so in Ireland as a student:
Not much is "must buy" - it depends on who you are. I found my greatest "buys" in Ireland were experiences, rather than things. So here's my list:
In terms of just merchandise? The Irish are a sweater people, given their climate, so Irish wool sweaters, scarves, etc. are decent buys. And if you're a slightly bigger dude, it is the land of the rugby shirt.
Irish lace is pretty famous, and some curtains or doilies or the like would make a nice conversation piece. If you are a "world music" kind of person, there will undoubtedly be CDs you can get of music that you really can't get any other way. Irish cheese is quite famous, but you may or may not be able to import it to where you're headed (check in advance.)
I would say that once you're there, concentrate on experiencing things. You may then want to buy something as a reminder (postcard, coffee table book, cookbook etc) or so that you can continue to experience something you enjoyed. Or if your intention is to bring presents home to those who didn't travel, choose the things that connect best to stories you can tell about your trip.