Tullow Parkrun starts on April 1st, at Rathwood.
Just register with parkrun, print out your barcode and go along to any parkrun event, Saturday mornings 9:30 for a free 5k.
http://www.parkrun.ie/tullow/
If you intend to go along regularly, they look for volunteers to share the work of marshalling - you'll see more about that on the link above.
I've run Johnstown Parkrun a few times - lovely setting, if a bit far away. Reports from Gorey Parkrun are also good.