"Red is the Rose" is a traditional Irish ballad of unknown origin. It's sung to the same tune as the famous Scottish song "Loch Lomond," and has a similar theme: the sadness of parting with one's lover.
I love this acapella version by the High Kings. The High Kings are an Irish folk group formed in Dublin in 2008. The band consists of Finbarr Clancy, Brian Dunphy, Darren Holden and George Murphy. By June 2016, the group had released four studio albums, two live albums, and two live DVDs. Their first three studio albums appeared at number three or higher on the Billboard world music chart, the first two went platinum in Ireland, and all of their albums charted in Ireland.