Not likely that far into the lough. From what I recall reading, those that were moored in Lough/River Foyle were towed well out to sea, off the north coast and scuttled/torpedoed/shelled there. They're estimating 50 foot in length for this object. My money is on an old surface vessel of some sort that's lain there for donkeys years.
Worryingly, your very close to the truth. The first submarine as we know it was designed by an Irish American(*) and was called the Fenian (something). It was intended to sink RN vessels off the east coast of the US.
Epic fail because * of a massive fall out between the relatives who designed and built the device, and the rest of the family who bought it. Without paying. Feggin Micks, eh?