The shear volume of stone walls in the West of Ireland is amazing. These simple but effective structures are used as field boundaries. The comparison with the East of Ireland is stark where the fields are larger and are usually divided by hedgerows instead.
Most of the stone walls in Western Ireland were constructed in the last 200 years to form field divisions and clear the ground. They were built without mortar and hence are called dry stone walls. To date the oldest known example of dry stone walls in Ireland are at the ‘The Ceide Fields’. These were erected 5,800 years ago, see Wikipedia page here.
In total there are estimated to be 400,000 km (250,000 miles) of stone walls in Ireland compared with roughly the same length again of hedgerows. While the shear quantity of stone walls in the West are astonishing, I would image that a well managed hedgerow provides a superior wildlife habitat than that afforded by these stark and bare walls.
Stone walls initially take much time and effort to construct compared with modern fencing. However they are much more efficient over longer periods of time when one considers how little, if any, maintenance is required. I have included a number photographs I recently took when visiting the amazing small island of Inisherr (The smallest of the Aran Islands). I think they give a flavour of the variety of different styles of dry stone walls that have been constructed in Ireland.