Place potatoes and parsnip in a pan with just enough boiling water to cover them, lid on. Bring to the boil and simmer for 15-20 minutes or until tender, drain. Place haddock in a deep frying pan with the milk and a small knob of butter and simmer for 10-15 minutes (timings will depend on thickness of fish).
Drain (save milk to mash with potatoes). Mash potatoes and parsnips with 200ml of the milk used to poach the fish and 2 tbsp fresh parsley. Mix together the mayonnaise and 2 tbsp fresh parsley. Mound the mash into the bottom of a shallow dish, top with fish pieces, add spoonfuls of the mayo, top with prawns and scatter with extra parsley & serve.
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