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What and where is connection to Rory Dahl O'Cahain?

I see only a vague of similarity between "Londonderry Air" and "The Young Man's Dream". Here are Bunting's tune and a few other early versions in ABC, so you can play/ display and judge for yourself. There are many later copies of the tune, and Thomas Moore used it for his song "As a beam o'er the face of the waters" in the first issue of 'A Selection of Irish Melodies'.

The 'Irish' song "Young Man's Dream" is an imitation of the English "Love's Fancy, or, The Young Man's Dream" or, "She lay all naked in her bed". (See my broadside index, ZN2339).

X:1
T:The Young Man's Dream
T:Bunting, 1796
Q:60
L:1/4
M:3/4
K:G
G/2A/2|Bc/2e/2d/2B/2|AG3/4E/4G/2A/2|B3/2 G/2 B/4A/4G/2|G2||(d/4e/4f/2)|g (g/2f/2) (f/4e/4d/4f/4)|(ed3/4)B/4 (d/4e/4f/2)|g g/4f3/4e/4d3/4|e3/2 e/4f/4 g/2f/2|e d/2B/2 (e/4d/4)(B/4A/4)|G A/4G/4E/4D/4 G/2A/2|B A/2G/2 B/4A/4G/2|G2|]

X:2
T:Young man's dream (Tytler's song)
T:Scots Musical Museum #126 (1788)
Q:60
L:1/4
M:3/4
K:Cm
G/2|c/2d/2e(d/2c/2)|(e/4d/4) (c/4=B/4)cG|c/2d/2ed/2c/2|(e/4d/4) (c/4=B/4)c3/2G/2|c/2d/2ed/2c/2|(e/4d/4) c/4=B/4c||e/2d/4c/4)|B3/4 A/4G(F3/4G/8A/8)|G/2F/2E/2D/2C|E/2F/2G((A/2G/2)|(F/4A/4) (G/4F/4)Ee3/4c/4|B3/2A/4G(F3/2D/8A/8)|G3/2F/4E3/4D/4C|E/2F/2 (G/2c/2B/2) A/2|G/4F/4 E/4D/4 C3/2|]

X:3
T:I dream'd I lay (By Robert Burns)
T:Scots Musical Museum #146 (1788)
Q:60
L:1/4
M:3/4
K:F
D|F/2 G/2AA|B/2 (A/4G/4)AD|F3/4 G/4 A3/2 (d/2c/2)|B/2 A/4G/4F2|F3/4 G/4AA|B/2 A/4G/4AD|F3/2 G/4A(d/2c/2)|B/2 A/4G/4F2||c/2 d/4e/4fe|d3/4 c/4d(c/4A3/4)|c/2 d/4e/4f(e/2f/4g/4)|(f/4e/4) (d/4c/4)d2|f3/4 e/4d(d/2c/2)|B/2 A/4G/4AD|F3/4 G/4Ad/4c/4|B/2 A/4G/4F2|]

X:4
T: The Young Man's Dream
T: Hime's 'A New Selection.... Irish Airs', c 1800
Q:60
L:1/4
M:3/4
K:G
G3/4A/4Be|d/2A/2BE3/4F/4|G/2A/2B/2e/2d/2c/2|B/2A/2G2::d/2B/2gf|e/2^d/2eB|d/2B/2gf|e/2^d/2 e3/2f/2|g/2f/2ed|B/2A/2BE3/4F/4|G/2A/2B/2e/2d/2c/2|B/2A/2G2:|]