Gifford quotes Roland McHugh who suggests a children's game as the source of Molly's rhyme "will you be my man will you carry my can". This can be confirmed now from a written source:
A singular child's play, descending from a time when such a feudal system was in force, is still practised in Dublin, parts of Ulster and elsewhere in Ireland. One child places his hands together, another takes them between his [as the lord took the vassal's] and holding them firmly, asks: "Will you be my man?” The other answers: “I will”. “Will you carry my can?" "I will." "Will you dig my grave?" "I will." "Will you fight my fairies?" [a corruption of "frays"] "I will." Then, the questioning child stoops forward as if to give him the kiss, but in the play, they merely puff breaths at each other. The holding of the hands is to prevent them from being used in defence, as commentators have supposed the lord held the vassal's hands until he was sworn, through fear of an act of treachery. If the child who is questioned answers in the negative, he is driven away with buffets, as doubtless would have been the fate of any recalcitrant vassal.
George Sigerson, History of the land tenures and land classes of Ireland (1871), pp. 92-3
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