On another web page, I found the following which claims to identify Spancilhill's author:

The author of "Spancillhill", Michael Considine, was born around 1850 and emigrated to the USA from Spancillhill around 1870. Some of his siblings came with him, but some stayed behind. One of his brothers, Patrick, died, leaving his widow to look after a five month old son called John.

Michael went to the USA with the intention of bringing his sweetheart over and for them to be married, but he never saved enough money for her passage. His sweetheart was "Mack the Ranger's Daughter" and not "Ned the Farmers daughter" as in the popularised version. She was his childhood sweetheart, Mary MacNamara.

Michael worked in Boston for two years or so before moving to California. He suffered from ill health for a long time. Knowing he hadn't long to live, he wrote the poem "Spancilhill" to send home in rememberence of his love. He sent the poem to his nephew, John, Patrick's son, in Ireland.

Michael Considine died sometime in 1873. Some sources say he was buried in Spancilhill, but others say he was buried in California. Mary MacNamara remained faithful to his memory and never married.

Pretty tragic, huh?

A folk singer tells the story of how he got the real words to this song. He was a at a friend's house and when the friend's eldery mother offered him the lyrics. A few years later, the singer was going to perform Spancilhill, and a gruff voice interrupted and told him not to sing it. When the singer asked why, the voice said, "because you don't know it." When the singer performed it correctly, the man was very surprised and pleased. He identified himself as John Considine, Michael's nephew, who had guarded the words for so many years.