Who is Policeman MacCruiskeen from The Third Policeman and what is their importance?

MacCruiskeen is a policeman. He is also a craftsman, who creates maddeningly absurd items. He made a spear with a point that extends six inches beyond its visible end, a musical instrument that plays frequencies beyond human hearing, and a series of identical nesting boxes that get smaller and smaller, until they are not visible even with a magnifying lens. Policeman Fox supposedly went insane after a lengthy visit with MacCruiskeen and his mind boggling creations.