Traditionally, mutton (an older sheep) is used in Irish Stew. While many American versions call for beef, that wouldn't really be considered an Irish stew, which is a simple meal of mutton, potatoes, and root vegetables simmered on the stove. Mutton is very hard to find (impossible where I live), so lamb is being used instead. Lamb is meat from a sheep that is less than 1 year old. Hogget is meat from a sheep that is 1-2 years old. Mutton is usually meat from sheep that are around 3 years old.