Old sheep. I think this is how the naming is broken down.
Lamb meat is from a sheep under a year old.
Hogget is meat from a sheep over a year old but under two years.
Mutton is meat from a sheep that’s over two years old.
Irish stew (Irish: stobhach / Stobhach Gaelach)[1] is any variety of meat-and-root vegetables stew native to Ireland. As in all traditional folk dishes, the exact recipe is not consistent from time or place to place. Common ingredients include lamb, or mutton (mutton is used as it comes from less tender sheep over a year old, is fattier, and has a stronger flavour, and was generally more common in less-affluent times) as well as potatoes, onions, and parsley.[2] It may sometimes also include carrots.