The singer "in search of my love" meets her and is warned. "'Touch me not, and approach me not near; I belong to this Rath, and the Fairy host here.'" He tries to hold her but she disappears.
"The original Gaelic of this song is attributed to George Roberts about whom, if he existed, nothing is known" (source: OLochlainn-More). - BS
In the earliest known phase of Irish mythology, the Sidhe (Aes Sidhe, the People of the Hills) were the remnants of the Tuatha De Danaan, who had been defeated and driven underground by the Celtic invaders. Later the name came to be used of any generic fairy or sprite -- but the first sense may have more meaning in context.
A rath was the Irish name for a fortification or earthwork. - RBW