A ruined windmill! 

I emailed some place nearby and received the following link. It reads, "circular tower on a prominent hill is probably the remains of a windmill." 

https://commons.wikimedia.org/wiki/File:Ruined_tower,_Corballis,_Co._Meath_-_geograph.org.uk_-_634790.jpg#filehistory

Lots of info there including geolocation. :)