Tesco Mobile have an economy tariff called Tesco Value which won't support MMS, GPRS or Roaming but allows international calls from the UK to a lot of overseas countries (as well as very competitive inland cross-network mobile and landline calls) and for Ireland at 20p per minute when calling from the UK, Virgin Mobile are very reasonable too for calls to the Irish Republic from Britain - 15p per minute.  Both of these networks tariffs are Pay As You Go - just buy a SIM and put it in your phone, if your phone's unlocked.  Many O2  Contract handsets are unlocked anyway!