The key to successful corned beef hash is in the boiling of the potatoes. So few people in kitchens can successfully boil potatoes that I think it is worth including in the advanced practical examinations one day.
Firstly, if you are using peeled potatoes then use them shortly after they have been peeled and not when they have been sitting in a bucket of water for hours. Cut the potatoes into pieces of roughly equal thickness. If you are using unpeeled potatoes – which will give you a superior flavour and which is what I do myself – then carefully wash and sort the potatoes so that they are more or less the same size. You will need to peel them after cooking of course.
Next, use your head when choosing a saucepan. If you use one that is too big you will need to put in gallons of water in order to cover the potatoes. This will take ages to boil. Pour on water until the potatoes are just covered and then add plenty of salt and a tight-fitting lid. Bring the potatoes to the boil, and then turn the heat down so that they are boiling very gently. When they are completely cooked – after about 20 minutes – drain off the water. Replace the lid and let it stand for a minute while any excess water evaporates as steam. Don’t leave the potatoes much longer than this or they will form a hard, dry layer that doesn’t mash well.
Skin the potatoes if necessary. Add about ½ teaspoon salt and several turns of the pepper mill. Mash the potatoes. Do not put in any butter or liquid until the potatoes are completely mashed. If you are just making mashed potato you could add butter, milk or olive oil at this stage and start beating it. For corned beef hash you have no such worries.