There seems to be plenty of questions on this sub about the Potato Famine, but I'm more interested in what would have been the Irish staple before the discovery of the Americas, and how the introduction of the potato changed the Irish diet.
Did potatoes allow more intensive farming (calories per unit of land area) and a higher population, what was the staple beforehand?
It is hard to be certain but sources suggest that Irish peasants ate a lot of grain, especially oats, and dairy before the potato.
Your second point is exactly right. Cormac Ó Gráda used army recruitment records to show that the Irish were comparatively well fed in the first decades of the nineteenth century. The downside of the potato's efficiency was that it allowed larger numbers of tiny holdings and destroyed any diversity in the diets of the poor.