Leighlinbridge Castle — known locally as Black Castle — stands on a grassy bank above the River Barrow, just downstream of the village's medieval stone bridge. What survives is essentially the lower half of a square Norman keep: a stub of masonry maybe ten metres high, with the remains of vaulting visible inside and the corner of an outer wall traceable in the grass.
The setting is the appeal. The castle was built in 1181 to guard the river crossing, replaced earlier wooden defences, and was sacked, rebuilt, and finally destroyed over the next five centuries. From the ruin you look straight onto the nine-arched bridge — one of the oldest functioning stone bridges in Ireland — and across to the village beyond. Access is free and the site is open at all hours.