Thanks for the tip. In the end we just flew down to Kerry and stayed there for longer, driving back to Limerick in a hire car.
They also parked in long term area but no big deal when its a longer stop.
Will do more research next time to check other options.
Slight correction to published parking rate I gave above - charges don't apply between 2300 and 0600 so effectively priced at 17 hours per day x 4 Euro = 68/day. This contrasts with 10/day at Kerry, Sligo etc.