If data is not a consideration (the OP did not mention data), then Tesco is the way to go. They are an MVNO or O2.
There effective rates are 10 cents per minute for national calls (due to a top-up bonus), and even cheaper for many international destinations. For example the US is 2 cents a minute, while Hong Kong is 3 cents per minute.
See:
Tesco Mobile :: International Call Charges
You can buy the SIM in any Tesco in Ireland, the card is fee with a 10 EUR topup.