Finally i can say this in irish!
real talk though crabs can't swim
You've never snorkeled in the tropics then
my thought exactly, crabs don't swim, and I'm Mexican so I have snorkeled in the tropics.
Why is the "-teen" after the noun? Is that always the case?
Why is it 'cúig dhuine dhéag' but 'cúig phortán déag'? Ní thuigim.
duine ends in a vowel. déag is lenited after a singular noun that ends in a vowel.
So are the nouns always written in singular when counted? "Fourteen crab" f.ex.
For the most part. There is more detail about the exceptions to the general rule in the Tips & Notes for the Numbers skill.