I had of course been to Fanad before but decided to make the long trip up again in order to try and find evidence of the original lighthouse that had stood on the spot. Bill Long's "Bright Lights" book says the current light was built close to the original light. CIL's official page for the lighthouse also states "Construction went ahead and a new larger and higher tower, close to the original tower was built..."
So there would surely have been some evidence of this tower. A circle of stones? A stump of a tower? I paid my fiver in and circumnavigated the tower three times but saw nothing conclusive. The line being given by the guides at the Tower was that no, the new tower simply was an extension of the old tower and when I said that I thought otherwise, they seemed very interested but said they had seen nothing that might suggest an old tower, apart from maybe the domed post above set on the cliff edge. However this seems a bit shorter than the "5 feet 9 inches inside diameter by three stories high-ground, first floor and lantern" mentioned on the CIL site!
I had also viewed aerial photos of the area, trying to identify the site. There did seem to be a circular stone structure to the east and down the cliff from the current lighthouse. However, this turned out to be the old quay wall, visible sticking out of the cliff above. A nearby old stone ruined eyesore turns out to have been the coastguard station.
Maybe one of the buildings on the current site was built on top of the original ruins? Dunno.
Old quay wall visible below and to the right of the lighthouse and, below, the view from above.
I have sent off a few emails but nothing back so far