Preheat the oven to 190C/170C Fan/Gas 5.
For the soda bread, grease a baking tray with a little butter.
Place both of the flours, the butter, bicarbonate of soda, salt and peppercorns into a large mixing bowl. Gradually stir in the buttermilk, using your hands to bring the mixture together to form a soft dough.
Shape the dough into a rough loaf and transfer to the prepared baking tray. Dust the top with strong white bread flour. Bake in the oven for 45-50 minutes, or until golden-brown all over and cooked through. (The soda bread is cooked when it sounds hollow when tapped firmly on the base.) Transfer the loaf to a wire rack and set aside to cool completely.
Meanwhile, for the celery soup, heat the rapeseed oil in a large, heavy-based pan over a medium heat. Add the onions and garlic and fry gently for 4-5 minutes, or until softened.
Add the potato and fry for a further 5-6 minutes, stirring regularly.
Pour in the chicken stock and bring to the boil, then reduce the heat until the mixture is simmering and simmer for 4-5 minutes, or until the potatoes are beginning to soften.
Add the sliced celery to the pan and return the mixture to the boil. Reduce the heat until the mixture is simmering and simmer for a further 5-8 minutes, or until the celery is just tender.
Stir in the parsley leaves and simmer for a further 2-3 minutes. Stir in the double cream and heat through. Remove the pan from the heat and set aside to cool slightly.
When the soup has cooled slightly, blend it in a food processor until smooth, then strain through a fine sieve and season, to taste, with salt and freshly ground black pepper.
For the poached eggs, bring a pan of salted water to the boil and add the large splash of white wine vinegar. Reduce the heat until the water is just simmering.
Stir the water to create a vortex, then crack the eggs into the centre of the vortex one by one (or one at a time if you prefer). Poach the eggs for 3-5 minutes, or until the whites have just set. Carefully remove from the water using a slotted spoon and set aside to drain on kitchen paper.
Just before serving, finely chop the reserved celery leaves. Drizzle the eggs with a little rapeseed oil, sprinkle over the celery salt, then garnish with the chopped celery leaves.
To serve, divide the celery soup among four serving bowls and float one poached egg on top of each portion. Serve with slices of buttered soda bread.