The story is of a steady decline towards the unsurprising end of terrible consequences. James Mason is quite astonishing as the soft-spoken hero, whom you just cannot imagine of doing anything as bad as scaring a child, let alone a robbery. Obviously, he does both in the film and much more. His endless odyssey which is much less about him than the various Belfastian characters towards him and - although explicitly denied by the filmmaker in a bizarre opening title card - ultimately about the struggle for independence in Northern Ireland (or any similar situation)
There were two or three moments in the film that didn't hold up for me as well as they should, but the crazy painter and his bizarre co-inhabitant made it all up - a bold selection of outright frightening men from the fringes of society. There is no need to decipher the symbolism (payoff might be even bigger) - it's simply enjoyable through and through.
As mentioned above, Mason is an extremely effective anti-hero, with his light frame, his haunting looks and there seems to be something eating away at him. Even when I watched him on a vintage TV show he radiated a kind of possession. Must be big fun to work with such an actor.
Study this for great character sketches (great film)