Toirdealbach is the end boss in the Toirdealbach's Tomb dungeon.
Make sure all the trash mobs are dead before starting.
Toirdealbach has a spell(Toirdealbach Poison) that cast a poison area 5 meters around her, anyone standing in it will take damage.She recast the poison every 5-6 seconds while it lasts 20 seconds so it will never fade from her. So the most common method of killing Toirdealbach is to kite her around the room. Whoever has aggro simply runs around the Pillar and everone else does ranged at her, when she switches aggro next player starts to kite.
At certain points in the fight she will summon healers (Shadow Specter) kill these as fast as possible.
The healers are spawned when she reaches 95%,45% and 10% health.