When this old, long-forgotten thread was resurrected the other day, I initially assumed that 'GUEST,lily' was just one of those slighty disturbed people who wander in here from time to time and post random nonsense; but I was wrong as it turned out. She was talking about a recent series of children's books by Helen Dunmore that feature an undersea realm called Ingo just off the Cornish coast. The books feature a boat called The Peggy Gordon, so it's pretty obvious where Ms Dunmore got 'Ingo' from. The word, anyway; using it for an undersea realm seems to have been her own idea.

I'd put money on that being the source of the (almost inevitable, nowadays) 'celtic connection' claim, and I'm pretty sure that that business describing 'Ingo' as some sort of Avalon is modern and made-up, though I'd be glad to hear any evidence to the contrary. Ingo certainly is found as a christian name, though; no dispute there.

The fact is that nobody knows what it means in the context of this version of 'Peggy Gordon', which was collected by Helen Creighton. The set she printed in Maritime Folk Songs (Toronto: Ryerson Press, 1962, 74-5) was a collation: text from Dennis Smith, Chezzetcook, tune from Mrs Edward Gallagher, Chebucto Head. The text is quoted without source information in thread Origins: Peggy Gordon and the DT file appears to be a cut-down form of that taken from a revival recording or possibly from Sing Out.

I don't know if any other traditional examples of the song mention 'Ingo' (or anything like it) at all, but I wouldn't be surprised if Mr Smith's were the only one that does. Chances are it's just a garbling of 'England' as already suggested, with no mysterious background at all. If Mr Smith understood anything in particular by it, Miss Creighton didn't say. 'Peggy Gordon' isn't a very old song as such (second half of the C19) though of course most of it is made up of much older floating verses from the common Anglo-Scots-Irish stock.