This is another daft claim. It is daft for the same reason as Cassidy’s explanation for glim is daft. When people borrow between languages, they tend to borrow individual words. Occasionally they borrow well-known and often-used phrases. However they never borrow random bits of sentences and they borrow the most basic form of the word. Cassidy claims that ‘beef’ as in ‘to have a beef with someone’ comes from the Irish phrase b’aifirt which means (if it means anything) ‘was a reproach’. On its own and out of any context like this, it really means nothing and it would be pronounced baffirch, which doesn’t sound much like beef.
The origins of the slang term ‘beef’ (complaint) are not known. There are various suggestions involving disputes between sheep-herders and cow-punchers in the old west or soldiers complaining about their rations in the Civil War. However, ‘beef’ doesn’t come from b’aifirt and that’s a fact.