First, you’ll need to find the key to unlock the door to the cave in the cliff. Use Odin’s Sight to locate the key, which can be found in the sea. Then, use the key to unlock the room in the cliff to reach the treasure hoard map.
This flying paper is found on the banks of the river in Durrow, southwest Meath. You’ll need to follow it along the buildings near the river heading northeast. When you catch it you’ll unlock the Pict Scheme (Torso) tattoo design.