A name given to the most important, and most wise and reliable mate; of Irish or Celtic decent, within a group of trippas....normally has bald head, and ginger beard.
By addressing the RishNish...you are also pledging legions and loyalties to him.
Hey bro .... look over there; its RishNish! Let's cross the road to him and give him a swirl. He good for tick too, ould reliable RishNish.