He thinks that he himself is the very stone that was hurled at the castle.
More Irish Proverbs:
If you don't want flour on your shoes, don't go into the mill.Irish
Little talk is easy to cure.
Irish
It's the quiet pigs that eat the grain.
Irish
God's help is nearer than the door.
Irish
Neither give cherries to pigs nor advice to fools.
Irish
Half a leap falls into the ditch.
Irish