Forts

Plus, the shape of the fort will often be affected by the topography of the land - e.g. boundary walls may follow the contours of the land in order to maintain an even height, or they will be located just where a steep fall of land starts, so that anyone attacking them has to climb up to them.

They're mostly pretty circular, aren't they? And, where they aren't, the commonest reason is that they have been partly destroyed, or partly overlaid by some other structure.

Also worth bearing in mind that they were built over a very long period of time, and in many cases for different purposes - some were livestock enclosures, some were dwellings, some were fortifications, etc. Some were repurposed from one use to another over time. All of these factors would account for some design variation.

I think I've just lost a post. Check it out here and see whether it is described as a fort.

https://maps.archaeology.ie/HistoricEnvironment/