but an Irish catholic can get with and marry a nonreligious Norwegian and somehow preserve their heritage?

your heritage isn't just biology either. It's your culture, your religion heavily influences your culture.

Indeed, and I would say that a white European Christian and a white European atheist would still have more in common than a white and a black, or even more so, a Muslim.

From those two different countries the only thing they'd have in common would be being white. And the catholic is now flushing his heritage away because their kid will be raised with a mix of the 2 cultures and not just thiers. The kid might not be religious either because the athiest parent might not want religion being taught to the child.

If that's something that matters to the Catholic, then he/she shouldn't marry an atheist. Cultural compatibility should be a primary consideration for a marriage partner.

but you wouldn't run up to them screaming about their heritage because you wouldn't care, because they're 2 white people.

It has more to do with the fact that I'm agnostic myself. I would raise an objection to any of my family marrying into or converting to Islam or an Eastern religion, because I feel that Christianity or at least rational secularism is part of the cultural heritage of the West. I want my ancestors to be white, Western, and culturally Christian, like me and my ancestors.

But even still, I would probably have less problem with someone in my family marrying a Bosniak than a black Christian, depending on the circumstances.