This appears to be an ancient Dugout canoe, not far from Devinish island in the main lake. There have been several found in the lake, and the Corrib river. There is a similar such canoe near Annaghdown.
The boat was not at all steady, in a moderate Northerly wind that was blowing right across the lake - so getting decent imagery was quite difficult. There were plenty of fish around the wreck too, and in fact the entire area was just a mass of fish.
You can see how the movement of the boat becomes superimposed onto the imagery here. To get the best sidescan response, due to the angle of the boat on the bottom I had to sail across the swell. Not ideal.
