I'm not sure this is true. I've previously heard the exact same thing referring about "100% beef" in the US, but mcdonalds explicitly says on their website that it isn't true [0], which I assume they wouldn't do if it actually were true. I can't find anything about "100% irish beef" , but I kind of doubt it's true unless you can find some source talking about it.

[0]: https://www.mcdonalds.com/gb/en-gb/help/faq/18916-is-beef-a-...