I've often had the sense that an unofficial geographic distribution requirement affects who becomes a finalist for the Booker. In some years with two or more really good Irish novels, one may have been sidelined because the judges felt they needed to honor, say, an ANZ book.

The distribution requirement could redound to Ireland's benefit in years in which the judges think they've gone too long without honoring an Irish author. Either way, you're right:

It's unusual to see two Irish novels in the running. I haven't read The Colony and appreciate your reminder of it. For others who are unfamiliar with it, here's a link to a good, brief review by Kirkus Reviews: https://www.kirkusreviews.com/book-reviews/audrey-magee/the-colony-magee/