Mac is gaelic for "son of". O' means "grandson of". Mc and mac are used interchangeably between people of Scotch or Irish heritage.
There is no such clan name as McCreacherie or McCreachery etc. The closest I have heard of is the Scotch name McCeachern.
It's my guess that they were making fun of his older age, as in Mac "Creature".... Not a malicious slander just making fun.
www.geo.ed.ac.uk/home/scotland/genealogy.html