This is well worth the short diversion off the Galway road. I was in the vicinity looking for Toorclogher megalithic tomb (again, without any luck, again) and pulled over to have a look at the modern christian cross. Just behind this is the souterrain entrance. What a pleasant surprise! It's quite well preserved, the opening in the mound leading into a corbelled chamber. From there a tunnel leads down. Where that leads to is anyone's guess, I hadn't got the bottle, or a torch, to check.
So, a souterrain in a mound, that's at one end of a hill in a parish called Seefin. Hmmm.