It's a sheep hole, (I don't know what they are called in the IOM)
But it is the only one I've ever seen in a wall built of vertically placed stone!
It is just a short walk on the coastal path out of Port Erin.
Meanwhile I met a farmer from the other side of the moors here in North Yorkshire (Farndale) and his local word for these holes was, "pop holes" which I'd never heard of before.