Low-grade sandstone-hosted thorium mineralization is recorded in the Carboniferous Northwest Irish Basin in the Republic of Ireland and Northern Ireland. The thorium is concentrated within bitumen nodules which are replacive within the sandstones. A source of thorium was available in a Caledonian granite in the basin watershed, and the bitumen was a product of hydrocarbons generated within the basin. The thorium occurs as thorite inclusions in the bitumen, and the development of the nodules was enhanced by the polymerization of fluid hydrocarbons by irradiation from the thorium. Thorium mobility may have been enhanced by the formation of organometallic complexes. Mineralization probably therefore occurred during hydrocarbon migration, dated as Late Carboniferous to Permian.