i'm using talktalk at the moment-all you do is dial 13636 to register and then either dial that prefix before all calls or route all calls via them, no contract, line rental optional and no monthly fee.
all local/national off peak calls charged at 6c up to one hour, and all calls are cheaper than eircom. think its good cos the monthly fee of 10€ some companies charge for equates to about 166 calls a month which is a lot of off peak calls to make.
i've used them as a trial and liked them so went with them for line rental too