I'm a bit confused as looking at the Dublin Airport website, they list 12 different agencies... What two agencies are you looking at?
There are several possible reasons for the price difference. It could be a different model of car. Check what is included (or perhaps more important, what is not included). Also check the excess policy and whether the car is picked up with a full tank of petrol and has to be returned with a full tank.
I don't know about Ireland but in UK there are a couple of companies that give very low quotes but then clobber you will, all sorts of excesses - particularly damage to the car when it is returned. (Green Motion and Drivalia which used to trade as Easirent get very poor reviews on Trip Advisor with many disgruntled customers because of this.) You can end up paying substantially more in the end.
Also be wary of renting from a third party as you have no control over the company you will end up with - it could be one of the 'dodgy' ones....