This page details how you can begin the Wrath of the Druids DLC, and how to get to Ireland.
Wrath of the Druids: How to Get to Ireland
The Wrath of the Druids DLC begins in your settlement, Ravensthorpe. When you return home you’ll hear a horn blowing, and Eivor will comment about who has turned up at the docks. You’ll need to walk there to find someone named Azar waiting for you, who comes with news from Ireland.
You’ll soon learn that Azar is a trader from the East, who has made Ireland their new home. Azar wants to establish a trading post in Ravensthorpe by request of the King of Dublin. However, they first need you to help clear a naval blockade which has been setup by trading rivals at the mouth of the Nene River, to the northeast of Ravensthorpe. This leads to the first quest of the DLC: Irish Trade, which you must complete before you can get to Ireland.