I've just found this thread trying to find the lyrics to "Fennario" which I heard for the first time on a recent re-issue of a 1971 album called "Daybreak" by Joe & Bing, on the UK label Rev-Ola
They give the song a Civil War context with the line:
"I would marry you, but you wear that coat of blue" rather than "your guineas are too few"
This fits in with Brian R's post from July '04 which I might as well quote in full here:

The original "American" version of Peggy-O, was popularized by Union troops during the civil war. Those that created it were members of the Irish Brigades mustered out of New York, which took part in the invasion of confederate New Orleans in 1862. From there they fought many costly small battles north along the Mississippi, burning southern plantations as they went. It is doubtless that is where the lyrics that refer the burining of the area, and "our captain is buried in the Lousianna country side" came from.

The ultimate genesis of the song is in Irish/Scottish folk music. "The Maid of Fyvio" with similar lyrics and harmony predated the US civil war by about 2 centuries.