The tunes are fundamentally the same, though they have grown apart with the years.  According to  The Fiddler's Companion, Oh Dear What Can the Matter Be (a.k.a. Johnny's So Long at the Fair) was first published in the British Lyre for 1792, and "was sung as a famous duet between Samuel Harrison and his wife, the soprano Miss Cantelo, at Harrison's Concerts, periodic events which he began in 1776".  It became, as a consequence, widely-known, and turns up in England, Scotland and Ireland in various forms, and, as was mentioned above, the Scottish variant under discussion was used by Junior Crehan as the basis for his jig Misty Mountain.

Malcolm