While making an approach to a private grass airfield in Navan, the Pilot experienced a 5 mph drop in his normal approach airspeed and the aircraft drifted left of centerline. The nose was lowered and full power was applied, however, this action failed to recover the situation. The aircraft struck a wooden post, touched down in a field, and then impacted into a ditch, which was located left abeam the threshold of the easterly runway (RWY 09). Both occupants self evacuated from the aircraft without injury. There was no fire.