Bryan, I would also strongly recommend an automatic! Although I've never driven in Ireland (I chose to take a RS tour last fall instead), I have driven considerably in England so have some experience with driving in that part of the world and on "the other" side of the road.
I'm considered a "professional" driver (emergency vehicles) and can easily use either manual or automatic transmissions. However, I felt that with the narrow roads, unfamiliarity with the location and all the other factors that it would be best to stay with an auto. If you get into any kind of traffic density in the cities, perhaps along with one-way streets and not knowing your way around, you'll be glad to have an auto!
I've been travelling these days with both a GPS and Michelin Maps, and find that's a good combination. In Ireland especially, some of the roads didn't appear to be well marked and also many of the signs were in Irish (Gaelic) with the english lettering spray painted out.
Be especially careful with speeds or you'll be meeting the Garda under "unpleasant" circumstances, and you might want to get an International Driver's Permit also, as that includes an excellent summary of various road signs and what each of these means. This is especially important for parking. Check the net for the protocols for roundabouts.
Good luck and happy travels!