GPS in Ireland....
We were there last May and did fine with maps. There are many tiny roads there, barely paved, and all seemed to be on the maps and well marked. Prepare to be confused because the road signs, and some maps, are in Irish/Gaelic and not English.
We rented a car in Shannon and I don't recall seeing GPS as an option. If I were doing the GPS thing, I would bring a handheld unit. There are hiking trails everywhere, most not well marked. Software is available for my old Magellan Map-330, so you shouldn't have a problem finding mapping software for your e-Trex. We found some really detailed maps here in the States.
You can also rent a handheld or road GPS easy enough on the internet. As I said, we did fine with map a compass and didn't miss the GPS.
Driving there can be terrifying!
The roads are about 3"
wide and lined with rock fences just high enough to destroy your mirrors, with no shoulders. You'll notice "mirror trenches" through the shrubs that line some of the roads. You'll get use to it about day 3. Buy the insurance!!!