Ornsay island is accessible on foot at low tide, though it can be pretty soft underfoot!
The Ornsay Lighthouse is actually sited on a smaller islet just off Ornsay, called Eilean Sionnach. It was built by the engineer David Stevenson, and came into use in 1857. It was automated in 1962, and then converted from gas light to mains electricity as recently as 1988.
Isle Ornsay Lighthouse was for a time the property of Gavin Maxwell, and is the subject of Chapter 6 of his book 'Raven Seek thy Brother' (Longmans, 1968).