Is that actually true? According to [1], McDonalds (in 2020) was buying EUR160e6 worth of Irish beef accounting for 20% of its beef sales across Europe. It would make quite a loss for them if they shipped beef into Ireland while buying that much locally and exporting it. I can't find anything that mentions this "100% Irish beef" theory.
[1] https://www.rte.ie/news/coronavirus/2020/0323/1124769-corona...