Why do some Irish last names have two capital letters?

I mean names like O'Brien, O'Neill, McDonald, McFarland, etc.


Looking for reliable sources.|||Mc is short for "mac", the Irish word meaning "son", so the name McX means "son of X". When there's an "O" in the name, it means "grandson of" or "descendant of". This is because the Irish word "贸" means "from".|||Mc, or Mac means "son of" so the actual last name would be what follows, thus the capitalization. Not too sure about the O though.|||I dont know. But I think its pretty neat and unique.