The island has been artificially formed by cutting across the peninsula or ros on which the fortress is built and from which it takes its name.
The channel, which is flooded by the waters of the lake, is crossed by a bridge, although it is usually dry during the summer months.
This bridge was formerly protected by a guard house and gates, which were closed every night, sentinels being posted at the entrance when the castle was garrisoned in later times.
The present ruins consist of a keep, with the remains of the surrounding bawn wall, which was flanked by semicircular towers at the corners. A spiral stone stair leads to the top of the keep, with doorways opening at the various floors.
The fortress is based on a limestone rock, and sustained on the land side by a buttress of masonry.
The peninsula contains 158 acres, and copper mines were opened on it in 1804, which were worked for four years, in which time £80,000 worth of ore was extracted. Water getting in stopped further work. It was clearly proved that the mines had been worked many centuries before, and a number of stone implements, locally called “Danes’ hammers,” have been found on the island.
The castle is supposed to have been built towards the close of the fourteenth century by the family of O’Donoghue-Ross, who added the appellation of their home to distinguish them from the family of O’Donoghue-More.
A modern barrack capable of holding some two hundred men and officers was erected against the keep, but when it ceased to be used Lord Kenmare had the unsightly erection removed.
There is a legend regarding a great and wise Prince O’Donoghue who possessed the secret of eternal youth, and under whose rule the land prospered greatly. It is related how that during a splendid feast at Ross Castle he rose up amongst the company and made a prophetic oration, recounting accurately all that the future years would bring. In the midst of speaking he walked over to a window (which is still shown) and through it he passed out over the lake. Upon nearing the centre he turned round and waved his hand in farewell to those behind, and, the waters opening, he disappeared beneath them.
On May morning he is said to rise from his watery grave and ride over the lake on a white steed, surrounded by beautiful women and youths. His appearance is looked upon as a sign of a bountiful harvest.