I agree, Malcolm. The only serious evidence to the contrary is Herbert Hughes's statement in "Irish Country Songs" (don't have the precise volume to hand)that he was told (in the '70s?) by an old man that "Johnny, I Hardly Knew Ye" predated the American Civil War.

Despite the usual run of opinion, the reference to "the island of Sulloon" does not guarantee antiquity.

Conceivably, Hughes's informant was thinking of the tune alone, (perhaps attached to "The Three Crows"!)- but that conjecture itself is worth nothing at this point.