Have you ever watched someone really good play Tetris? As soon as a new block appears they have spun it to the correct orientation and shifted it to fall perfectly into the ideal place. Watching experienced dry stone wall builders is the same.
A dry stone wall is only a single stone deep (thick?) and uses no cement. All that holds up a six foot high wall is friction between the stones and the skill of the builder as they place each rock so it interlocks with its neighbours. I’ve built stone walls in the Burren and, believe me, it was a long, slow process. I probably handled each stone a half dozen times before finding the (more-or-less) correct place to fit it in. The guy I was working with could pick up a big rock from the pile at his feet, _already_ holding it in the correct orientation, and place it unhesitatingly into the developing wall in the ideal position. Every. Single. Time.
Dry stones walls appear fragile but are remarkably resilient. Key to their survival is how level the top edge is and if you look closely at well maintained walls you’ll see that they are ‘topped’ with successively smaller stones so that the upper edge is level. This is because dips in the wall are perceived by cattle as points of weakness and will get pushed and rubbed against until a few stones fall and then a gap big enough to push through develops.
Some walls have gaps and stiles built into them. In the Aran Islands, there will be ‘doors’ in the walls that are filled with rounded stones. These can be easily ‘opened’ with a push (all the round stones fall) and then can be reassembled after the cattle have been moved. You will see smaller ‘doors’ low in some walls that were made to allow sheep through.
If you start looking more closely at these stone walls you’ll begin to notice different styles. Sometimes there are regularly spaced tall vertical stones, called ‘mother stones’ that are used, particularly for higher walls. Sometimes there will be a lower, horizontal tier, supporting an upper two thirds of vertically oriented stone. Whole books have been written about all the different stone wall styles in the West of Ireland. Next time you’re here, see how many styles you can spot.