Ritchie, that hole is at the back of my house. as kieran said it does take a lot of water to work, the sluice has to b pretty much washed out.
to get there- take the 1st Leixlip exit after lucan. when you get into the village, take a left at the lights. go up the hill and take the first right (its right before the church n school). and its at the bridge at the botom of that hill