Can you tell us your location? If you can get RTE1 and 2 then you should be able to get TV3 and TG with out too much trouble. Chances are they are transmitted from the same mast and at the same power, but a local installer would have much better knowledge. For the expense of getting an outside aerial a satellite dish might be a better option. You can get a dish and box from the likes of Maplin [broken link removed] for about €130 (DIY install) and it will give you all the BBCs (7?), ITVs (4?) and a load of other channels most of which are not much good.
Towger
Edit: You can also buy the aerials and dishs from ElectroPlus . They are basicly trade, but have no problem selling to joe public.