At the bottom of the steep hill in Kilmacrennan you take the road to the left and St. Columba's sits just along that road on the right. The graveyard is just across the road from it and inside the gates is a gravestone part of the inscription on it reads: "The first guests of the cemetery". (Close up of that stone can be seen below here).
Kilmacrennan is where the Donegal Saint, St. Colmcille (Columba) commenced his eduction in the 6th century AD.