My point was that the relative value of goods could change. Even if someone in Ireland cannot buy as much rice as someone in India maybe they could buy more pot

My point was that the relative value of goods could change. Even if someone in Ireland cannot buy as much rice as someone in India maybe they could buy more potatoes, because potatoes are easier to grow in Ireland while rice is easier to grow in India. If we want to use food as a metric, we would probably want to use some abstract measure of nutrition (maybe $/calorie).