Okay, so to recap. There was a high and a low lighthouse built in 1781. In 1818, these were replaced by a new High and Low lighthouse. The two 1818 lights are still standing - the high one is featured in the two photographs on this page, whereas the low one is still operational, is painted white and is built much further down the cliff face.
Of the two 1781 lights, the octagonal High light still remains, being used as very expensive holiday apartments. But where exactly was the old 1781 Low Light which was demolished in 1818 to make way for the 1818 High light, above and below?
Clues were somewhat scarce - apparently it was similar in shape and proportion to the 1781 High light only smaller. So it was octagonal. I'd be looking for octagonal foundations.
Second clue - the new High Light (above and below) was built very near to where the old Low light had stood.
Third clue - both sets of lights marked the path for boats to sail between the India and Arklow sandbanks and reach Wicklow Harbour successfully.
Armed with these three clues, I set off in hope.
Of course, 196 years of grass meant there was no hope in locating the foundations. My fond hopes of scratching away a few weeds to reveal a perfect octagonal stone foundation were dashed.
On the way home I pondered it more and figured a flaw in my reasoning. The lines between the two sets of lights wouldn't be parallel to each other or they'd be pointing to different places in the sea. Idiot!
And at home, researching some more, I came across a web entry saying the foundation stones of the old light could still be seen under the newer lighthouse, next to the path. Of course, I hadn't looked on that side of the lighthouse. You can probably see it better in the top black and white picture. There's a square patch of flat land just to the right of the path before it disappears around the bend.
I reckon this must be it. But it will have to wait for another time to prove or disprove this!