"Brigit sought Saint Senan's holy well. The vikings did not know of it, and it was the one place on the island left undefiled. Shallow, it trickled from the moss to pool in a tiny rock-lined basin...She knelt and sank both hands into the water." (pp. 76-78)
We can see where she did it because the first image is:
-copied from here.
The second image shows the monks' tower that the vikings besieged and attacked and the ruin of the church where Brigit cared for Halldor's son. The tower and ruined building are described as:
Round Tower and St. Senan's Church on Scattery Island
-also copied from here.