St Mullins is a lovely village located near to the River Barrow. The area near the river is very well maintained with lots of green space, picnic tables and public toilets. Down along the river there is a nice café and from here you can walk the Barrow Way back to Carlow if you have the energy.
Access to the water is best done from stone steps just across from the café. You can stand at the bottom of the steps here and launch yourself in. The water is cool and dark and refreshing. The Barrow is a large river and you would want to be careful swimming here if there is any kind of a decent flow to the water.
All in all a lovely spot for a swim.
From Carlow take the M9 and then exit 6. Follow the R705 to Borris and then you take the R729 which leads to New Ross. At the village of Glynn watch out for a right turn signposted St. Mullins.