This page of IGN’s Assassin’s Creed Valhalla wiki contains info for obtaining the Trophy/Achievement, The Legend of St. Patrick, in the Wrath of the Druids DLC. This is obtained if you kill the only snake in Ireland. This trophy/achievement is not locked behind any quests and is basically available upon your arrival in Ireland, but you should at least complete the Blood Bond quest to get yourself settled in Dublin first.
The only snake in Ireland is hidden in an abandoned church in the Meath territory. The church is about 400m north of the Lisdurrow Trading Post, in the marshlands between Inchroe and Tara. If this is your first time visiting the place, it’s also the location containing a Cursed Symbol. Check out our guide for all the Ireland Cursed Symbols.
Enter the church. Inside the small room, you’ll find lots of jars and crates. The snake is inside one of the jars! Just start swinging your weapon to break the jars, and continue swinging to kill the snake before it even has time to strike. The snake doesn’t respawn after it’s been killed. Congrats! Ireland is snake-free!