Great thread which I will add to sporadically. For years I've kept a little notebook exactly for this purpose.

However, to make it really useful what is needed is some sort of Excel file so all of the tunes can be given in alphabetical order including variant titles and searchable so that all those related can be pulled out as a file, plus midis etc attached. (I must add that my own technical abilities are minimal.)

We can argue till the cows come home about whether 2 tunes are related or not, but for now just flagging up those that present obvious similarities would be a start, as is being done.

How about using Roud numbers to identify the songs more precisely? I could help with this.

Toorali-ooralie-addy (Adieu to all judges and juries)
Caroline and her Young Sailor Bold
Bells of North Loo (Bell-ringers song)
The Alderman's/Nobleman's Lady

'Derry Down' rivals Villikins for the most used tune, certainly if we look at 18th century ballads.
The earliest use I have is 'King John and the Bishop/Abbot' then 'A Cobbler there was' both 17thc or earlier. The most obvious usage on the folk scene was 'The Dreadnought'.

Swaggering Boney is Weel bred Cappy, The Old Yorkshire Tyke and on broadsides 'The Chapter of Kings'

The following also have similarities.
All round my Hat, The Nobleman's Wedding, Green Willow, Spencer the Rover, Spanish Ladies ( minor), Early one morning.

Shule Agra, Let the Bullgine run.

The Amphitrite, Banks of Sweet Dundee

And of course:
Baa Baa Black Sheep, Twinkle twinkle, Little Nut tree, Incy Wincy Spider, I'm a Little Teapot, etc.   All for now.