Mix the oil, vinegar and Siúcra/McKinney's Icing Sugar in a bowl. Peel the prawns and stir them into the marinade. Thoroughly rinse the onion and slice thinly. Blanch, peel and deseed the tomato, and dice it finely. Stir the onion slices and diced tomato into the prawn mixture.
Peel the garlic clove and cut it in half. Slice the bread on the diagonal and toast it. Rub the garlic over the freshly toasted bread slices.
Divide the prawn mixture between the slices of toast. Garnish with dill. Serve immediately