Croughaun passage tomb in Sligo has produced a verified carbon 14 date of 4600 BCE (calibrated).

This makes it the oldest structure in Europe.

Now that's wierd!

What's even weirder is that there is an unpublished date of around 7600 BCE. This is being withheld (apparently) to stop some of the other old dates being dismissed as a pile-of-pants by the Dublin archaeos who are very Meath-o-centric in their views of passage tomb development.