Around Portadown if you were a hardened drinker and a bit of a looney bin, it would not be uncommon for the term 'Hammerhead' to be used to describe you.
I wonder is this a term used outside the area, or does it stem from this:
'Mick vividly remembers the lighters and the lighter men; his parents were lock keepers at lock No. 9. He talks enthuiastically of the characters he remembers, of Hughie Fox who captained the Fox, of a frightening man named Davidson but known as Hammerhead. This poor fellow died of exposure, alcohol and malnutrition.At the inquest into his death in Newry Workhouse, a witness stated he probably hadn’t had his boots of for the last 15 years.'
http://heritage.iwai.ie/waterways/newry_canal1.shtml