Sadly despite it being epic for waves, Ireland is also known for its weather. Summers are brief, the water temperature rarely gets over the mid teens in centigrade, so you need a wetsuit year round if you want to get in the water.
Summer time and you may just get away with a 3/2mm but are probably going to be more comfortable in a 4/3mm. In the winter though things get cold, really cold, and so a full steamer up to 6mm with built in hood is the best option.