So no ',' after 'but' in Irish?
There wouldn't be in English in this sentence either.
My inclination would be to add a comma after “eats”, and that would probably exert some influence on me to add one after sí also.
Do they use comma's in Irish?
Yes. Generally in the same place English would use them.
If you want to add a superfluous but still grammatically correct comma, you can place one before "but", not after. Apostrophes aren't used to make plurals either :-)
It's odd that they're comparing two people being different, but not using emphatic.
I was just thinking the same thing. It ought to be "Itheann sise ach ólann seisean."
wow that was REALLY confusing