The Courthouse hub is the practical place to start in Kells. It gathers local relics and a copy of the Book of Kells, and the Market Cross outside is one of the town's most visible medieval survivals.
The cross has had a rough modern life: after centuries at the town crossroads, it was knocked down by a reversing bus in 1996, repaired, and moved under cover. That odd detail makes it a good introduction to Kells, where serious monastic history sits right inside an ordinary Irish town.