Put half of the potatoes in the bottom of a large heavy saucepan. Cover the potatoes with the lamb and season. Cover the lamb with the onions (and the other vegetables if you are using them) and the rest of the potatoes then add the rest of the seasoning.
Add the thyme and the water (or stock). Bring slowly to the boil and skim off any scum that rises with a slotted spoon. Cover and simmer very gently for 3-4 hours. Allow to cool completely and then chill. Remove any excess fat from the top of the stew then reheat gently.Adjust the seasoning to taste and add the chopped parsley.
Serve a bowl of the thin, delicately flavoured stock first as soup, followed by ladlefuls of the meat and potatoes with more juices as ‘gravy'.
The classic Irish stew dates from the days when the Irish had very little to eat except potatoes and a few bones so if you want to get the really classic flavour, the ingredients list should be very short and the cooking time very long. However, as with all classic recipes, extra ingredients can be added at the discretion of the cook so I have given them as options.