The Instructor and Student departed Weston (EIWT) on a navigational exercise in a northwest direction towards Donegal. Near Athboy, Co Meath, the engine oil pressure dropped off without warning and the Instructor decided to terminate the exercise and route back to Weston. However, just north of Maynooth, the engine started to run roughly and to lose RPM. The Instructor decided to carry out an immediate forced landing at a nearby private airfield rather than attempt to reach Weston. This landing was safely executed and the crew exited the aircraft without further incident. There were no injuries.