The Aircraft departed Runway (RWY) 26 at Kerry Airport (EIKY) for a transatlantic flight to Gander (CYQX), Newfoundland, Canada. Shortly after departure, as it was passing 1,800 ft, the aircraft contacted Shannon low level control. When requested to confirm their squawk5 and passing altitude, the Flight Crew read back the correct squawk and stated that they were level at Flight Level6 Two Hundred. The Controller was concerned that the aircraft had levelled out at 2,000 ft and was tracking towards high ground in the Slieve Mish area of Co. Kerry. The controller then instructed the Flight Crew to climb to Flight Level Three Zero Zero and to expedite their climb until they were through four thousand feet. The Flight Crew complied with this instruction and the flight proceeded without further incident.