Who is Michael Gilhaney from The Third Policeman and what is their importance?

Michael Gilhaney visits the police station to complain about his missing bicycle, which Sergeant Pluck stole himself. Gilhaney is over fifty percent bicycle by riding it too long and exchanging atoms with the cycle. Pluck relates the story of how Gilhaney took advantage of a schoolteacher by tricking her into riding his bicycle while he rode hers.