Is столовая an irregular noun?

Having trouble understanding this example from my course;

Кошка в столовой -- Столовая is a dining room, so I'd expect the prepositional singular to be столове. How does it end up with -ой as the ending? Is it an irregular noun, or is there some other rule at work I don't understand?

Thanks,
Lindsay

Столовая is declined like an adjective because it's an adjective which became a noun. It was столовая комната first.

Спасибо Marcus!