Tube-like trace fossils occur within dolomitised Visean pelmicrites in County Fermanagh, Northern Ireland. The pelmicrites are associated with algal laminated carbonates and evaporites which imply deposition in a coastal sabkha. The tubes exhibit a preferred orientation caused by minor current systems. Thin sections and scanning electron micrographs show that in life the tubes were characteristically bilaminar, with an internal sheath of organic material and an organically cemented tube wall. This structure resembles that of modern polychaete worms. Comparison with a Bahaman polychaete suggests that the Visean worms inhabited a subtidal algal mat. The diagenetic history of the tubes is one of progressive dolomitisation and is closely linked to the presence of organic substances in the walls of the tubes and the mucus sheath which lined the tubes during the worm's life. The presence of the organic matter may have been a prerequisite for the formation of the dolomite observed in the tubes. Ichnofossils, diagenesis, dolomitisation, sabkha environment, Polychaeta, algal mats. Carboniferous, Visean, Northern Ireland, Fermanagh.