The Castle That Guarded Two Kingdoms

Carlingford Lough is a sea lough. It runs inland from the Irish Sea between the Mourne Mountains on the northern shore and the Cooley Mountains on the southern. Today it is the border between Northern Ireland and the Republic of Ireland. Eight hundred years ago, it was the contested frontier between two Anglo-Norman lordships — and before that, the boundary between the ancient kingdoms of Ulster and Louth. Control the lough, and you control the crossing between north and south. Both sides knew it.

In the 1230s, Hugh de Lacy — one of the most powerful Anglo-Norman lords in Ireland — built a castle on the northern shore to guard the entrance to the lough. He called it Greencastle. Directly across the water, barely two miles away as the crow flies, another fortress already stood: Carlingford Castle, built by his rival John de Courcy decades earlier. Two castles, staring at each other across a narrow stretch of water, each daring the other to move.

What Hugh de Lacy built was no minor fortification. Greencastle rose to four storeys — a massive rectangular keep of local granite and limestone, designed to dominate the shoreline and command views in every direction. From the upper floors, sentries could see ships entering the lough from the Irish Sea, watch the approach roads from Kilkeel (Cill Chaoil, ‘the narrow church’) and the Mourne coast, and — most crucially — keep an eye on Carlingford Castle on the opposite shore.

The castle was not just a watchtower. It was a royal castle — a seat of administrative power, a court, a garrison, and a symbol of English authority on the contested northern frontier. It controlled access to the lough, regulated trade and fishing, and projected military power across the water. In a landscape where loyalty shifted like the tide, Greencastle was a statement carved in stone: we are here, and we are staying.

In 1315, Edward Bruce — brother of Robert the Bruce, King of Scotland — landed in Ireland with a Scottish army. His aim was nothing less than a second front against the English crown: while Robert fought in Scotland, Edward would take Ireland. He marched south through Ulster, burning towns and besieging fortresses, and in 1316 his forces attacked Greencastle.

The assault on Greencastle was part of a wider campaign of devastation across the Mourne coast. Bruce’s Scottish soldiers swept through the region, and the castle — for all its thick walls — was damaged. But it survived. It was repaired, re-garrisoned, and continued to serve as a military and administrative base for decades afterward. The keep that stands today bears the scars of that period — patched masonry, altered doorways, the quiet evidence of a building that has been fought over and rebuilt and fought over again.

By the late medieval period, the Anglo-Norman lordship that had built Greencastle had fragmented. The castle changed hands repeatedly — seized by Irish chieftains, reclaimed by English garrisons, damaged, repaired, and eventually abandoned as the strategic value of the site diminished and newer fortifications were built elsewhere. The roof fell in. The floors collapsed. The walls stood open to the Atlantic weather.

But the stone endured. Today Greencastle is in state care — a protected monument, open and free to visit. The keep still rises to its original height in places. You can trace the line of the upper floors in the stonework. You can stand in the great hall and look up at the sky where a vaulted ceiling once stood. And you can walk to the shore and look across the lough to where Carlingford Castle — its twin, its rival, its mirror — still stands on the opposite bank, now in a different jurisdiction, a different currency zone, a different country entirely.