On a cold and wet day in March 2018, a van crammed with equipment and Scottish and Irish archaeologists rolled up alongside Navan Fort, a prominent hilltop settlement in Northern Ireland that’s not far from the modern border with Ireland. A ringed enclosure that sits on a tall hill in the countryside, Navan Fort is easy to pick out. It was one of the ceremonial centers of pre-Gaelic Ireland, and even has a visitor’s center today. But that March, the archaeologists were seeking remains that didn’t meet the eye.Lo and…