"Are you on the bus?"
Translation:An bhfuil tú ar an mbus?
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.