North or South?
I believe North is considered part of our system, it has a UK STD code.
South are a nation to themselves.

Also, what phone system / company would you be using?

Without knowing, I'd guess that Eircom like BT would have high enough charges (~15c/min) for all UK landlines that they'd probably include these; well, there is no separate category in their call calculator.

If you use cheap calls providers, you'd find that they were more expensive than landlines, probably either a national rate call there or approaching the cost of UK mobiles

eg Telestunt has UK 0845 and 0870 via Irish 0818 numbers, for which Eircom charge ~8c, 5c off-peak

http://telestunt.ie/accessnumbers.php#u

If you're there for a while, you might set up an account with http://www.call13434.ie/index2.php which might look familiar ... (5c for UK 0845)