The first of the Children of Danu you'll meet in Assassin's Creed Valhalla: Wrath of the Druids is The Wren.
Being the subject of the quest Potion of Blood, you'll encounter The Wren as part of the story.
The Wren - First Clue
To complete the first clue you have of the Wren, you must go to the beach north of Port Auley, to the abandoned house next to the dock.
Port Auley is to the north-west of Connacht. It is a fairly easy place to find, as it is a useful trading post for overseas trade from Azar to Dublin.
The Wren - Second Clue
Next, you'll be sent to investigate a ritual stone in Lough Gara.
The Wren - Third Clue
Niamh, the poet, is The Wren. Head to the town of Rathcroghan, south of your position.
A golden triskelion will mark her position. Be mindful that the town is a Distrust Area, so blend in and assassinate The Wren.