When I told him we were going to one of the highest cliffs in Europe, my brother asked me about the geology of Slieve League - "I have no idea" was my reply.
But I did take a picture at the back of the mountain - it still doesn't help me, but I'm sure he'll know what those black and white rocks are.
If you're really keen about the geology, you may be interested in this little article I came across - apparently Slieve League is connected to the Appalachian mountains, from a long long time ago before the Atlantic ocean separated them! Isn't that fascinating! I always loved my plate tectonics!
As for the rocks themselves, here is what I found in that article: "In the area of Slieve League the
dominant rock type is the Slieve Tooey Quartzite (of which both Slieve League and the neighbouring Slieve Tooey are constructed). It is a fairly pure quartzite formed in the shallow waters of the continental margin of Rodinia. Below the quartzite lies the Glencolumbkille Limestone, a unit of carbonate sediments deposited in the warm seas of Rodinia, now metamorphosed to marble. "
So there you go!