Could the base culture of Erain be changed to Gaul? The south was generally more heavily affected by the Gauls, while the Briton tribes generally settled the north more heavily (though both had tribes in both places, such as Brigantes in the south, and Uillii in the north). It's not important, too much really, but it would affect, I think, the temples that would possibly be built there, and it just seems more likely to me, in the south, a temple at Menapia (the Menapii were mainly Gallic) would be to a Gallic god, not a British one.

Resources, perhaps silver in Cunnacht, as there were silver mines there, Patrick notes them. While this would be hundreds of years after the setting, it's still notable that silver deposits were present, and they should have the chance to mine them. Also, there is green marble in the west (the hills of Connemara), but don't know if any resource would represent that.

In Erain, the most notable feature was really the abundant farmlands, as Erain controlled more of the fertile center of the island, but also had the larger iron reserves, and had more available hunting lands, so perhaps furs or something similar? Or maybe linens, though the south grew famous for linens a bit later, I believe.

In Caledonia, is wool a resource? If so, I'd have that in Caledonia, and with the Dumnones and Ordovices. I'm not sure about the rest, to be honest. I'm not really sure about the wool 100% either, and Erain's up in the air for me too. Cunnacht though, I'd give silver, and marble if there is a resource for it.