Monaghan started off in a grannie mystery show, Hetty Wainthrop Investigates.
He went on to bigger and better things, LOTRs, as one of the four hobbits. To be honest I should think most people believed this was his debut, as the before mentioned show weren't much cop.
He's now appearing in the new international smash hit tv show, lost. Although his character in this show seems nothing more than a whiner.
I preferred him as the funny hobbit with his comedy partner merry, rather than this supposed Brit rock star. Which the story strangely resembles the relationship of the Oasis, Gallagher brothers.
I reckon his best market is comedy, he doesn't seem to have much of a dramatic presence or features. His character in lost isn't a stereotypical british guy, which is refreshing, but more to do with the clever and original writing than the actor.