"Tá inneall díosail sa bhus."
Translation:There is a diesel engine in the bus.
Is" díosail" not rather a noun in the genitive case in Irish? sort of working like "engine of the diesel".
And diesel just an adjective in English in "diesel engine"?
One of the purposes of the genitive is to allow to use a noun as the attribute of another noun - in other words, to use a noun as an adjective, you generally use the genitive form of the noun.
It's not really an either/or situation.