|Possible Irish water monster|
One of the things that has struck cryptozoologists about these lake monsters is often the lakes they occupy seem to small to sustain a monster. I feel an answer to this lies in the eel section of the name.
Sometimes these horse-eels have been reported crossing the land and it has even been averred they are equipped with legs. The answer to the small lake problem may be that, while they are sometimes seen in such lakes, they do not stay in the same lake all the time, but journey from lake to lake. The reason that they are not seen doing this more often is because many of the Irish lakes are in areas of uninhabited wilderness unfrequented by visitors. Moreover, it is known that eels can cross land when they need to.
My suspicion is that we might have here an unsuspected species of large eel, whose head when held up in the water resembles a horse's profile, which is able to cross land and which may even have small legs. Some of them may be large, but not necessarily all of them. If you see only their backs in the water, perhaps broader than those of known eels, it could give the "upturned boat" effect.
There are. of course, some Irish monsters to which this explanation would not apply; but it's worth thinking about, so start thinking out there.