From The Grateful Dead lyrics pages (?!) With quote from Pete St.John (who wrote it in 1888, according to another site google found for me!)

The song was written in 1979 and recorded by Paddy Reilly, whose best-selling single launched an album of the same name. However, over the past 17 years more than 400 cover versions have been made with conservative estimates on single sales put at five million. The song was based on a true story of the fate of one young couple during the Irish famine.

The song tells the story of Lord Trevelyan who brought a supply of corn back from America in a bid to battle starvation during the potato famine in the mid-nineteenth century. Unfortunately it was Indian corn too hard to be milled, so useless. However, local people thought it would save them and so broke into the stores, were arrested, and subsequently deported to Australia.

"The song could be about anyone Scots, Irish, English. It is about poor innocent people and how they are victims of natural disasters. It's easy to say why it's been so popular in Glasgow because in 1846, the year the song's set, over 150,000 Irishmen, women, and children fled to the city where many were treated with generosity. But I've heard the song sung everywhere from San Francisco to Melbourne."

Also mentions that there was a call for the song to be banned at Parkhead where Glasgow Celtic supporters sing it to wind up the Rangers supporters...