Going on a 2-3 day local tour will answer most of your questions. Are you going to do any cooking? The issue that gave me the most worry for my 2 week tour was cooking gear and food. I spent weeks researching stoves before deciding to build an alcohol burner. I ended up never using it.
The weight you're going to carry is going to come down to comfort...how much you require personally (number of clothes changes, toiletries, reading material, souvenirs, etc) and how much repair and emergency equipment you carry to keep yourself from worrying.
Without getting into the unique challenges Ireland may pose, I'd aim for 20-25 pounds. A lighter load will of course put less stress on your legs and bike. If your bike fits properly and your rear wheel is strong and you won't have to worry about your largest potential problems.