Where It All Goes Down
Ireland; Haven; the Fowl Estate
The setting in this book is quite literally all over the map—or under it, as the case may be, since because Haven exists underneath all the surface continents. Once we get past all the flying and the traveling to Ho Chi Minh City though, Ireland is where everything goes down.
Artemis shows up in Ireland because, well, he lives there, and Holly shows up in Ireland too because Ireland is the "old country," the most magical spot in the world. Most of the later action in the book happens within an eight-hour time bubble dropped over Fowl Manor and the grounds, though fortunately this doesn't leave us longing for a change of scenery.
Think about the two major settings—Haven and Fowl Manor—together. Both are old, but one was built out of necessity when the fairies were driven underground, and the other was built with money and for family legacy. Fairies have to be invited into Artemis's setting, but there's nothing stopping him from walking in Haven's front door (assuming he could find it)—and because of this, Haven is less like an actual haven than Fowl Manor, which in turn makes Fowl Manor a symbol of Artemis's expansive power.
But then again, time-stop gives the fairies a way to turn any place into their home turf and control the setting in both space and time—though Artemis robs them of this ability at his house. And when he does, Artemis single-handedly turns the most magical country in the world into the number one danger zone for fairies, casting doubt on the future of fairy life (and possible setting up the next book in the series).