In my experience "fornenst" is more likely to be used in rural areas rather than in a city like Belfast (for instance, my country-born grandparents used the word frequently but it was never part of my Belfast-born and bred parents' vocabulary), but it certainly wouldn't be misunderstood by any one who grew up in Belfast. "Fornit" is a new one on me.

I think the reference to "the Group" is a giveaway that it came from the pen of James Young. For many years the Group Theatre in Belfast was the artistic home of Young.