The proportion of salt to water is very important. Although the fish or piece of fish should be just covered with water, the aim is to use the minimum amount of water to preserve the maximum flavour, so therefore one should use a saucepan that will fit the fish exactly. An oval cast iron saucepan is usually perfect.
Half fill the pan with measured salted water and bring to the boil.
Put in the piece of fish, just covering with water, and bring back to the boil. Simmer gently for 20 minutes. Turn off the heat, allow the fish to sit in the water. Serve within 15 to 20 minutes. Meanwhile make the Irish Butter Sauce.