From experience, most of the mapping companies have reasonable representations of Northern Ireland, but there is NOTHING worth talking about for Southern Ireland, unless you live in Dublin. The mapping of the rest of the country is
useless, and not worth the money, autoroute being one of the worst, the latest version I got, (2005) had poorer maps than the 2002 version.
Much of the responsibility lies with Ordnance Survey Ireland, a state controlled monopoly, who have so far not made the digital data of Ireland available at an acceptable price to the companies that supply the data to the software integrators. As a result, Ireland ends up looking even more like a banana republic than it already is, and the impression it gives to foreign tourists, especially from the States, where they're used to street level GPS can only be imagined.
I don't see it changing in a hurry, unless someone manages to put something very provocative under Ordnance Survey, as a state controlled body, they have no incentive what so ever to do anything that might be deemed as useful in providing a service to the thousands of people that visit Ireland each year.
You're more likely to see good GPS mapping of Outer Mongolia before you see good coverage of the Republic.