Is tiresome an adjective

Adjectives don't have common or proper designations. Even when used for to describe a proper noun, an adjective is not capitalized, for example 'The tired Mr. Dickens set down his pen.'

The only exception is when an adjective, such as tired, is part of a name or title, for example 'The Tired Traveler Motel' or the Irish musical group 'Tired Pony'.