As the picture has nothing distinctive, it could be hard to find the exact location but I am sure every member here will be happy to help.
A few years back, I went on a week boat trip along the lough Ern, so I had a look at some of my pictures, and nothing looked similar. I will check with my friend's pictures too and let you know if I find anything. I was first thinking of Devenish Island due to what could be a wall on the bottom left corner; unfortunately, the scenery around the island does not seem to match the view on this picture.
May I ask where you found this picture? Any information will help narrow the search area as County Fermanagh
has over 150 islands, many not far from Enniskillen.