Buy the magazine Construct Ireland or look up Joseph Little architects. He has written some outstanding papers on the differences in the various methods of insulating properties.
http://www.josephlittlearchitects.com/ These papers have been published over the past couple of issues of the magazine.
There is no doubt that external insulation methods are the best retro-fit solution. They are expensive anywhere from €70-€140 per m2 depending on the size, number of storeys, complexity of design, external finish to match etc.
When you take into account the hassle factor of adjusting your internal wall sizes, window boards, skirting, radiators (under windows) etc. the refinishing and painting of your internal walls I personally recommend external insulation, first, for all of my customers. Their choice and budget will decide the selected method.
Before you ask, I subcontract all the work, so no vested interest in a particular product.
Outsulation has been guided to me at approx €120 per m2. An average 3 bed semi-d having 100m2 of wall to be treated and then your grant allowance refund.
Check Joe Littles papers, the return in your investment is clearly explained.