Dont know that guy but it is one of the oldest depictions of an Irish chieftain's event all be it from an anglo norman perspective. So one must use what is there. Nothing earlier but sometime I have to go through the manuscripts just to be sure.

In Ireland beef was a high status and high income item therefore mutton.
But there is a quality in mutton that can not be replicated so to get historically with it....therefore...

But of course substitute what you will....

Irish stew however should always be lamb- British stew would be beef as in the roast beef of olde england. I am continually surprised at the number of Irish american events that substitute beef. It is these days cheaper so maybe they are rationalizing.

Strange also that on greek indepenedence day here in baltimore just after tomorrow one finds souvlaki in pork and chicken only no lamb.
Insane....

I like to recommend adding pepper and sauteing the onions a bit longer till brown rather than translucent to ofset what folks dont like about lamb.

When in doubt use lamb.

Conrad