actually it is a ref quoted by A W Moore and is not a Manx name but one found in Irish annals (see www.manxnotebook.com/manxnb/v05p020.htm) which is the Moore reference mentioned by Nigel as along with the later J J Kneen possibly relying too heavily on the Irish clan name. This not to say such derivation is incorrect - Moore suggest = Tadg's son.
In Manx records found is earliest in 1408 as MAC KYG