This priest speaks of the Mass rocks in this excellent sermon:
I tried to set it to start at the sermon, but it didn't.
Sermon is from 14:24 - 31:40.
He speaks of how the Catholics would have lost the Faith without those clandestine Masses. He quotes a researcher who said "These rocks are the symbol of Ireland's Catholic heritage. They are what kept the Faith alive for almost 2 centuries of the Irish catholic penal era; if Gaelic communities hadn't gone to the Mass rocks, the Catholic Faith would have died out." Fr. Johnson goes on to say: "And surely this is what our overlords are counting on."