Heat the olive oil in a 9-inch, heavy bottomed, frying pan and add the potato slices carefully, because the salt will make the oil splatter. Try to keep the potato slices separated so they will not stick together. Cook, turning occasionally, over medium heat for 5 minutes. Add the onions and garlic and cook until the potatoes are tender. Drain into a colander, leaving about 3 tablespoons of oil in the skillet.
Meanwhile, in a large bowl, whisk the eggs with a good pinch of salt. Add the potatoes, and stir to coat with the egg. Add the egg-coated potatoes to the very hot oil in the skillet, spreading them evenly to completely cover the base of the frying pan. Lower the heat to medium and continue to cook, shaking the pan frequently, until mixture is half set.
Use a plate to cover the frying pan and invert the omelette away from the hand holding the plate (so as not to burn your hand with any escaping oil). Add 1 tablespoon oil to the pan and slide the omelette back into the skillet on its uncooked side. Cook until completely set. Allow the omelette to cool, and then cut it into wedges. Season it with salt and serve with Ballymaloe Country Relish.
Serve warm or at room temperature. Delicious for lunch for a picnic.