The original lyrics as recorded by Harry Champion for the Columbia Graphophone Company in December 1911 clearly state "somebody said to MacIntyre" so MacIntyre was told by somebody not to "let 'em in".
Back in the early days of folk clubs, around 1960, HMV issued a compilation LP called "A Jug of Punch". One of the tracks on this LP was "When the Old Dun Cow Caught Fire" sung by Bob and Ron Copper (with banjo accompaniment!!). They had learned the song from Bob's father Jim Copper and somewhere along the line their chorus had become "somebody SHOUTED MacIntyre" As recordings of folk songs were pretty rare at this time every song on this LP was soon done to death in the folk clubs and the tradition of the audience shouting out MacIntire soon established.