fistswit

(n.) Irish:  the strong silent type who responds to witty insults with violence.

"Mind your gob, Ken.  Yah Niall's a spacer, but hiz a narky fistswit."

Translation:
Be careful, American visitor to the run-down, coastal pub. While it true that Niall's devoid of thought, in is also true that he is a large, irritable fellow who returns punches for witticisms that he perceives to be directed at him.