Beware Some 'bad' language below.
Maybe we might run with 'cac' all the same . Some research has come up with this website. http://www.lincolnu.edu/~focal/backinst/focal094.htm
And I also came across this curse!
"Thalla is déan cac mhór bhuidhe!"
'Away and do a big yellow shite'
(shite to rhyme with 'white' is often used instead of shit in Ireland)