Green’s Dictionary of Slang

(Irish) a fairground showman.

1910		Thomas MacDonagh Songs of Myself in Regan Irish Writing (2004) 385: For there again were thimble men / And shooting galleries, / And card trick men and Maggie men / Of all sorts and degrees .
1952		B. MacMahon Children of the Rainbow 142: A maggieman mounted in wall-eyed guard over his spy-holed barrel.