I think in English buttocks is a plural noun - so this would be translated just as "fat buttocks". As opposed to bottom which is a singular noun so would be "a fat bottom". I realised too late after I had moved on to the next question but will put a report in when I see it again.
I'd probably use "(o) mo chreach!" For a somewhat stronger cursing Gaelic often uses religious terms rather then body-related ones, so you can probably encounter "an Donas = the Devil" as often as, if not more often than, "hell!" in English, including in expressions like "an Donas (something)! = the damned (something)!" or "Donas fhios agam = Damned if I know".
This is not sensible English. In normal UK English usage, you would say a fat bottom, or a fat bum. We hardly ever use buttocks in everyday speech. If you really wanted to say buttocks, it would be just 'fat buttocks', except that no one would normally say that. I think bottom is a much better general translation for ton, if you are trying to relate to normal speech.