I suppose this must be the right site for this sad story (so many nearby forts to choose between, but this has a souterrain). It was recounted by the boy's father, Pat James.
The Fairy Fort.
At a place near Ballina called Curroy, there is a very fine fort. It has been and is still owned by a very old family called James.
One day the eldest of the James' was picking nuts in the fort. Without knowing he suddenly found himself at the mouth of a large cave. He entered, and came to a kind of stone door. This he opened and entered into a beautiful furnished room. There were tables, chairs and other articles of furniture in it. The boy soon had all the furniture in his own house, which was quite close, to the fort. Next morning the furniture was gone. Soon after, the same boy was thrown from his bicycle and died.
From the Schools Collection of the 1930s, part of the National Folklore Collection and now being digitised at duchas.ie.