I would attend a couple of open mikes at the place you're going to make your debut at - just to see what the general ambiance is like. See what kind of material is being performed and then you'll be able to judge better whether or not you should go a capella or whether it would be better to find someone to accompany you. If it's a lot of young "singer/songwriters" with guitars they might not go for a capella. But if it's traditional folk (especially if there's a lot of Irish songs) it would probably be fine.