Preheat the oven to 180°C.
Sift flour, bicarbonate of soda, cream of tartar, and salt into a bowl and stir in sugar.
Gradually mix in buttermilk until a soft dough forms.
Knead dough lightly, shape into a ball, place on a floured tray, and cut a cross on top.
Bake for 30-35 minutes until golden and hollow-sounding when tapped.
Melt butter, then stir in mashed potatoes, salt, and flour to combine.
Knead the potato mixture on a floured surface until smooth.
Roll dough into two large circles 0.5cm thick and cut each into quarters.
Fry potato farls in a non-stick pan for 3-4 minutes per side until golden.
Roast sausages in a tray for 25-30 minutes until cooked through.
Season tomatoes and mushrooms with oil and pepper, then roast for 10-15 minutes.
Fry black pudding slices in oil for 1-2 minutes per side until crisp.
Dry fry bacon rashers for 6-8 minutes until golden-brown.
Fry eggs in hot oil for 1-2 minutes until yolks are just set.
Serve farls, sausages, pudding, bacon, tomatoes, mushrooms, and eggs with buttered soda bread.