The Morrigan’s Altar is found northeast of Benn Uava in Ulster.
Offer 25 Luxuries at The Morrigan’s Altar
This offering altar requires that you offer 25 Luxuries. Luxuries are a new item introduced in the Wrath of the Druids DLC. They are a form of currency found in petty loot chests across Ireland, but can also be collected by unlocking Trade Posts. You’ll want to unlock Port Auley in Connacht, as it is a Trade Post that generates Luxuries, which can then be picked up from your trade post chest in Dublin.