I can't speak for how people in the Gaeltacht use tóin, but I can tell you that tóin basically just means 'bottom' -
tóin na farraige = bottom of the sea.
tóin an bháid = bottom of the boat.
If anything, therefore, it's even more polite than 'bum'. (isn't that 'fanny' to the Americans? - a whole different discussion there!).
Yes, it is (which was another thing we had to teach that group of kids we took to London in 2005...not to call their "bum bags" "fanny packs" (which is what they're usually called here)).
Though it's actually a little old-fashioned anymore. You're more likely to hear "butt" (which was considered INCREDIBLY vulgar when I was in elementary school, but has lost that impact today), or "backside" or "hiney." My daughter's kindergarten teacher in North Carolina referred to it as "the back porch."