"Are you on the bus?"
Translation:An bhfuil tú ar an mbus?
Yes! The use of eclipsis and lenition depends on the dialect, but eclipsis is used in the standard.
The an mbus is dative and an bhus accusative, isn’t it?
Also: is it a typical way to say that in Irish, or would that mean that you are really on the top of the bus? Would sa bhus/sa mbus be less/more/equally natural thing to say?
an bhus is dative in Ulster Irish. The accusative is always the same form as the nominative, and follows the same rules for lenition/eclipsis, so the accusative is an bus - chaill mé an bus.
De Bhaldraithe includes a number of examples of ar in his entry for "bus", and even translates an English phrase "in a bus" to ar ... bhus.
bus2, v.i. F: Téim ar bhus. ("go by bus" might be the closest English equivalent)
"A twopenny ride on the bus" - aistear dhá phingin ar an mbus
"I don't suppose I have been in a bus for twenty years" - ní dóigh liom go rabhas ar aon bhus le fiche bliain.
The NEID also prefers ar an mbus.