The case concerning the Flower of Magherally is much as stated by Malcolm Douglas (RIP) above. The earliest reference is to Dominick Maguire and he is the source for the JIFSS report and for Sam Henry's. Despite Colm O'Lochlainn's reference to ballad sheets my research into this aspect of song circulation in Ireland did not turn up any sets of this song on such sheets.

As far as The Star of the County Down is concerned this is what I said in another thread:
"According to Colm O'Lochlainn "Songwriters of Ireland in the English Tongue" The Star of the County Down was undoubtedly written by Cathal McGarvey 1866-1927, a native of Ramelton, Co Donegal who lived most of his life in Dublin.

McGarvey also wrote the Devil and Bailiff McGlynn."

Rosie McCann is invoked not because she was a local beauty but because the name McCann rhymes with Bann.