Two Story extensions are a bit trickier, have a look at this
www.environ.ie/en/Publications/DevelopmentandHousing/Planning/FileDownLoad,1586,en.pdf
for a bit of further detail (i cant post is as a link for some reason, you'll have to cut and paste).
The best advice that I can give you is to generally in these cases:
1. Talk to a qualified and experienced Architect or Engineer who knows what the planning restrictions, and:
2. Get the design of the extension right and worry about whats exempt or not afterwards. Many people have a sort of phobia about planning applications, and end up with a totally unsuitable extension, or one which is only half as useful as it could have been, as a result.
To answer your particular question, you may only construct 12 sq.m. as an extension on a first floor, keep it 2m from a boundary, and the height restriction will vary depending on the height and design of the rear of the house. This advice, is subject to conditions.
www.studioplustwo.com