I bet they come in big bags!

But in all seriousness, I didn't know what to make of the few I saw in The Burren whilst out walking there one time. When I enquired someone told me they were shepherds' lookout points - and perhaps they are today: perhaps they weren't yesterday. The Burren ones are about 4ft in diameter at the base and around 7ft tall with nicely pointy-rounded tops.

There's a nice polystyrene one up near the source of the river Liffey in the Wicklow Mountains: it's very convincing if you're more than 20 ft away from it. I presume it's left over from some filming - they're filming the new King Arthur film up there soon.