Educate yourself thouroughly first. You should find a good jeweller in Dublin to show you stones and read up on them. Decide what you want in terms of C's and price. You need to be fairly expert to know what you are doing, the cert is far from all-important.
Overall I would advise against buying a stone from a dealer. Customer care (have you thought about having it set?) and after sales care are very important when the diamond is not an investment. Most jewellers will look after you well after you have bought from them, money well spent IMHO.
PS. Carat is the least important characteristic of a diamond, a well cut fair quality 70 pointer can be astonishingly beautiful while a poorly cut 1.5 carat diamond looks like a piece of a broken windscreen.
PPS. Steer clear of the "Perfect Cut", "Hearts and Arrows" branding.