The other major site we visited on Inis Mór was the Wormhole (a.k.a. "Poll na bPéist", a.k.a. "Serpent's Lair").
A near-perfect rectangular void in the landscape, it's as if all the forces that carved regular lines in rocks all over the island got together for one final group effort.
I loved it but should warn how hard it is to find: that last comical picture really was the only signpost we saw (same goes for the half dozen people we tapped for help along the way).
It's an enjoyable enough hunt but I could only agree with my (non-Irish, big fan of Ireland) wife that..."you guys really aren't good at tourism".