This expression is applied to a friendly match-maker, introduced by the young man, to relate to the parents of the girl of his heart – in glowing terms – what a desirable match his friend would make for their daughter.
He praises his good looks, his good prospects, etc., and tells the old people what a splendid husband he would make.
He is also supposed to act as guardian over the engaged one while the man himself is away from home.
The Dooinney Molla has another part to play, and that is of lover by proxy. Sometimes he plays this part too well, and marries the girl himself!
(source: Manx Yarns by AE LaMothe (1905); painting by Bartholome Esteban Murillo ‘Two Women at a Window‘)