I don't know how long the think I'm using will last but you should try it while you can:
Voipcheap (in beta testing at the moment) currently allows you to dial landlines in Ireland, UK, France and a couple of other European countries, and Australia for FREE FREE FREE at any time of the day or night.
All you need is a headset.
The "catch" is that to register you have to make a payment of £5 sterling via Credit card or Paypal. Not so bad though as you can use up this credit to ring for instance Irish mobiles - at 14p a minute or 22 cent approx - no great deal but not much different than Eircom, or use it to ring any countries not covered by the free calls.
Having registered, ALL your Irish, UK and the other specified countries are free even at peak time.
I signed up 3 weeks ago and have not made one landline call from my house in that time. You'll save a fair bit especially if you use the phone during the day. It's too good to last forever I guess!
www.voipcheap.co.uk
Oh, and apart from ringing landlines, as with other VOIP services, you can VOIP from one voipcheap user to another for free obviously.