I have a Garmin 60CSX which I have loaded with both topo and road gps mapping software. Works fine, but it is not as sophisticated as a new road only GPS. No voices telling you when to turn, it just beeps and gives you instructions on the screen. Works fine for me, but it is nicer if you have a co-pilot to read what the GPS tells you to do when it beeps. Memory is so cheap you can load a whole lot of maps.
Don't know about Ireland road maps, but I imagine they exist.
It is nice to be able to use the GPS to get TO the trailhead, and the switch over to the topo maps when you are on the trail.
60CSX is a bit old now, perhaps Garmin has a better version in some of their newer GPS.