The Burren is a karst landscape in County Clare, Ireland. The Seventh Sacred Site is located here; it is the portal tomb Poll na mBrón—"the cavern of sorrows". According to Magnus Bane, the Burren is a good place to raise a demon.
The Burren was the place where Sebastian Morgenstern chose to create the Infernal Cup and to perform the ceremony of transforming the remaining loyal followers of Valentine Morgenstern and those unhappy with the Clave into Dark Shadowhunters. After the ceremony, they were later joined by the New York Conclave who faced Sebastian's new Endarkened army in battle.
Simon Lewis fought his way through the battlefield and handed the sword Glorious to Clary Fray to use it on Sebastian—to sever his bond with Jace as well as to kill him by burning all the evil, and his life force, inside him. Clary, however, was unable to get close enough to Sebastian and instead ended up using the sword on Jace. The battle ended when Sebastian and his army fled.