Who is Martin Finnucane from The Third Policeman and what is their importance?

Martin Finnucane is a thief and a murderer. The narrator meets Martin on the roadside, and Finnucane, who has no respect for life plans to kill him, even if he has no money. Martin and the narrator have a connection in that they both have a wooden leg, and in many ways, Finnucane is the narrator's doppelganger, a reflection of himself.