Thompson: Hyde published a version of they lyrics of Casadh an tSugáin in Love Songs of Connacht (1893), so he was aware of the song before he wrote the play.

https://archive.org/details/LoveSongsOfConnacht/page/n83/mode/2up
An Súisín Bán, the White Coverlet, pp 74-75

The play, with a translation by Lady Gregory, is also viewable on the Archive https://archive.org/details/casadhtsgino00hydeuoft/mode/2up