Hogg's Jacobite Reliques (1821) gives two versions of the song - 'modern' and 'original'. Both of them give 'young chevalier'.
It may have nothing to do with the song but Chambers Scots Dictionary defines the word as 'a favourite son', which fits into the sense of the song.
On the other hand - from the chapter on Kelly From Killane in John McLaughlin's 'One Green hill:
"Shelmalier (or Shelmaliere), Siol Maoluidhir ('place of the descendants of Maolughra'), East and West, are baronies to the north and west of Wexford town. Forth and Bargy are also barony names and lie to the south and west of Wexford.
The term 'barony' was the old name for the territorial division of counties and fell into disuse about a hundred years ago. Within the thirty-two counties of Ireland there are two hundred and seventy-three baronies, then further subdivided into parishes and local areas called 'townlands', originally based on land owned by extended families. In 1798 regiments and other military groupings were often distinguished by the name of the barony that they came from, e.g. the Scarawalsh units, the Shelmalier Cavalry, the Gorey Yeomen, etc."
Jim Carroll