Ireland is a massive area you can explore in Assassin’s Creed Valhalla with the Wrath of the Druids DLC. You’ll be able to undergo a new quest to assist a high king and unlock trade posts in the country to exchange exotic goods. When you’re visiting the Ulster region, there’s a locked door underneath the synchronization point that you can access if you find the key. Inside the locked door are some exotic goods and resources you can use to upgrade your armor and weapons.
The door is directly underneath the synchronization point, at the base of the tower. You want to go around the back of the ruined structure and use your Raven’s sight to find the key. It will be under a small pile of rubble inside the interior area of the tower.
The only way to expose the key is to blow up the rubble. On the second floor of the door is a blocked door, but you can cut this flimsy door down with a few hits from your melee weapon. Inside, there are several explosive jars. Grab one of them, bring them outside, and toss them on the pile of rubble. A fire will erupt, and it should go away after a few seconds, giving you direct access to the key.
You can now return to the front of the fortress and unlock the door to grab your well-earned resources.