I have just read another version of Cliodhna's death in:
Pg. 185 Sketches in Carbery
In this one, Cliodhna is not sleeping on the beach, but chained to a
rock when she died. She is still a foreigner. I wonder if she might
have been a slave and sold as an unwilling concubine to the local
king, who then punished her by chaining her to the rock to die.
It makes more sense seeing that it is the rock and wave hitting the
rock that is named after Cliodhna and not the beach. So perhaps it was
not a tsunami but murder most foul?