While flying at an altitude of approximately 2,500 feet, north-east of Gorey, Co. Wexford, with a student and instructor on board, the instructor observed hot engine oil flowing from the instrument panel of the Cessna 150M aircraft, and that the engine oil pressure gauge had no pressure indication. A precautionary landing was completed into an agricultural field. The occupants evacuated the aircraft unaided. Subsequently, it was identified that an unapproved repair had been carried out on the engine oil pressure gauge supply pipe, and that this repair had failed. There were no injuries or fire.