You cannot visit Okinoshima, and that is the whole point of it. This small rocky island sits in the Genkai Sea, about sixty kilometres off the coast of northern Kyushu, and it has been a sacred site for around sixteen hundred years. Sailors crossing to Korea and China used to stop here to pray for safe passage, and they left treasure: mirrors, gold rings, glass from Persia, offerings that archaeologists have found in the thousands, so many that the island is sometimes called the Treasure House of the Sea.
Today the island belongs to the Munakata Taisha shrine, which treats it as the dwelling of a goddess. The rules are strict and well known. Women have never been allowed. Men who visited used to strip and purify themselves in the sea before setting foot ashore, and nobody was allowed to take anything away, not even a blade of grass, and nobody spoke of what they saw. Since 2018 the island has been closed to the public entirely, apart from the one priest on duty at a time and a single ceremony in late spring, and it was inscribed as a World Heritage site in 2017.
For a rider the practical story is on the mainland. The coast near Munakata, northeast of Fukuoka, is pleasant riding: low hills, sea views, small fishing ports, and the main shrine buildings that make up the other components of the site, including a remote worship hall on the island of Oshima, reached by ferry. Fukuoka is a big city with fuel, food and an excellent ramen culture, and from there the roads along the Genkai coast are easy, quiet outside the summer weekends.
If you want to look at the island, the best idea is the distant one: from Oshima's viewing hall on a clear day you can just make out the shape of Okinoshima on the horizon. That is as close as most of us ever get, and the effect of being so near to something so firmly closed is more powerful than many places that are open to everyone.