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#uIf 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)