The specimen on the left is a 'single' wall and is quite common on the island. These are about 3.5ft high.
I'd been on Sherkin several times and never seen this one, until one day on the ferry to birdwatch on Cape Clear island the weather forced the ferryman to take us via the steep eastern side of the island. My eye attracted to a wall on the distant hillside I decided a closer look was in order.
The dog, by the way, is mine and she's Jilly a border collie X retriever.
And so several weeks passed before I had the opportunity. The wall runs for just a couple of hundred yards along the top of steep cliffs. It was built to keep cattle from straying over the cliffs.
So far as I'm aware these are the only pictures in existence of this rather unusually built wall, which still functions to keep cattle out of harms way.