"MacBeth, son of Bedha, son of Finlay" You are right in that Mac was used but not as a surname. Surnames weren't used until later. Macbeth MacFinlay (anglicised) was the son of Finlay. The personal first name Macbeth as I understand it did not mean 'son of Bedha' and was in fact not even really a patronym but was a first name meaning something like 'son of the church' or 'holy person'