Don't understand why Cyclezealot is so negative about cycling in Ireland. Sure, the roads in and out of the big towns are very busy but once clear of the towns it's possible to find small secondary roads that have little or no traffic. Earlier this year I cycled for two weeks in the NW, mostly Donegal and Fermanagh, and had a wonderful time staying in hostels. I plan to return in 2011 and do the Burren and Connemara region.
Apart from the Scottish Highlands I can't think of a better, more interesting and friendly country in which to cycle than Ireland.