The Ancient Irish had some interesting rituals. One of them appears to have been the "Killing" of boats.
This particular vessel, an immense boat, probably larger than the Lurgan Boat in the National Museum (its hard to be exact as some of it remains buried under rocks), has been sunk, possibly with some rocks already aboard, and then had rocks dropped onto it.
The method of doing this was probably quite sophisticated, as the rocks are remarkably accurate - the precise orientation of the boat had to be known, and was probably important.
This particular vessel is now very deteriorated, and it appears to be close to disappearing altogether. It has not had the benefit of being covered by the lakes protective silt and the erosion of millenia has taken its toll.
Corrib holds several vessels that have been sunk like this, and they are not isolated to this particular part of lake. A vessel to the north of Doorus was sunk in an identical manner.
These boats, and the mysterious ritual surrounding their sinking will likely remain a mystery. Hopefully one day the reasons will be unravelled - because these craft hold much of the story of Ancient Ireland. To date it has not been investigated.