Shelmalier is indeed a place, the 15km or so in a circle around Wexford town, I think. It's also mentioned in Boolavogue, another song about the 1798 Rising:

At Boolavogue, as the sun was setting
O'er the bright May meadows of Shelmalier,
A rebel hand set the heather blazing
And brought the neighbours from far and near,
And Father Murphy from old Kilcormac
Spurred up the rocks with a warning cry
"Arm, arm," he said, "for I've come to lead you,
"For Ireland's freedom we'll fight or die."

You can follow the route of the Rising if you use Google Maps and go from town to town as mentioned in Boolavogue.

I always heard the line in Kelly the Boy From Killane as "Say what wind from the south" rather than from the sun. 'Beavers' are the beaver hats popular both in America and in Europe in the 18th century. Bag*e*nal Harvey was spelled with an 'e'.

I'm told that there are still Kellys in Killane, and they're still a tall family.

Wexford is a county with a rich mix of people, whose ancestry includes Huguenot (Protestants fled from genocide in France), Palatine (Protestant Germans fled from French genocide), English and Gael.