Last Tuesday, I showed you a large stone-built letter 'E' from a remote hilltop in West Cork. If you figured out why it was there and what it's for, well done!
The 'E' is part of a larger series of letters: EIRE. The letters along with a marker number were built during World War II around the Irish coastline to alert Allied and German bombers that they had reached Ireland - a neutral country.
Some of the EIRE signs are still extant and this one is located near Toe Head. Here are all four letters (click for the full view):
And here's an image of one of the signs from the air:
Finally, here's a clip from the BBC series Coast on just this subject.