Wring the grated potato out in a clean tea towel to dry it out a little. Discard the starchy water.
Add all ingredients except the buttermilk to a mixing bowl and stir well to combine.
Stir in the buttermilk to make a thick batter.
Heat a little oil or butter in a frying pan over a medium/low heat, then add tablespoonfuls of the boxty batter and fry until golden and cooked though, about 5 minutes per side. Flip and cook the other side.
If making batches, keep them warm in the oven.