In chapter 2 of Jonah in PUL's Cork Irish Bible, the translation for "casting lots" is "Chrannchur"

To me, this looks like possibly a compounded word of aspirated "Crann" and "cur" so this word is something like "Tree-put." Am I on to anything here or is this just a coincidence?

Yes, it's a very literal translation of "casting lots" where lots were sticks or pieces of wood which would literally be cast.