The following link is more or less the standard ranking system for world universities.
You have to question the importance of ranking though, and how it will affect you. The USA for example take the rankings into account (i.e. what school you went to) in terms of employment etc., but I'd imagine in Europe that unless it's for academic studies or profession, there's not a whole lot of industry that take your university too much into account, a "good" university is preferable, but not in such detail as picking between the ones you've listed, which are all excellent.
In terms of Irish Universities, Trinity usually ranked around 50th in the world, and UCD around 90th in the world, but since the recession, and mainly due to a lack of funding, they've slipped to about 100th and 150th respectively.
Also remember that these rankings are very generic, but the school you attend in a university might be ranked much higher / lower - like Trinity for certain business disciplines would not rank as well as UCD (imo) and UCD would be much lower ranked if it was based on their Arts disciplines for example (again, imo).