Green’s Dictionary of Slang

(Irish) a general term of abuse.

1997		Indep. on Sun. 9 Mar. n.p.: Yeh little git. You told me you wanted to write your bleedin’ memoirs, and I believed you, didn’t I? Foolish ate-your-bun dat I am.
2002		O’Byrne Files: Dublin Sl. Dict. 🌐 Ate-your -bun n. Term of contempt.