20x20 will require planning permission. The maximum size without planning is 25sq meters. (about 15x18'). There are also other restrictions. If it is on a farm I seem to remember something about feed sheds being larger without permission, but have a look at your local county council website for specifics on the relevant planning laws.
Also, yardmaster sheds go to a maximum of about 9' x 16', as sold by Argos. They are great once assembled, but I'd hate to try and dis-assemble and re-assemble one.
I'd say you are going to have trouble locating a second hand one, but there are a few metal shed manufacturers in Ireland, either google or golden pages will give them to you.