A quick, short term, solution, which usually works with UK modems anyway, (havent tried it with phones), is to open the RJ11 junction box on the wall, i.e. the socket where you plug in the cable. In there you usually see 4 terminals with only 2 connected to the phone line/extention. Get 2 pieces of bare wire and connect the 2 on the left together and connect the 2 on the right together. Now either an Irish or UK connection should work. The main reasons the UK and Irish systems are incompatable are (a) the plug and (b) they use the 2 inner wires of the 4 core connecting cable and Irish phones use the 2 outer.
Caution, dont do this with wet hands, or do anything silly like licking the terminals (though this is a handy way to check to see which 2 wires in a bunch carry the phone signal), as you will get a bit of a tickle (48V DC). Wont kill you but will make your nose itch.