Sean (John) Bergin and Sean McDermott lost their lives on 19 April 1921. Unfortunately I have no details of the incident.

The place names are Lough Glinn, indeed about 6 miles NW of Castlerea. If it's an artificial lake, it's big one, over a mile in length. Brackloon, is about 3 miles NE of Castlerea. Bellanagare, is about 6 miles NE of Castlerea.

(I have Brendan Shine singing the song, as well as another anon version. Possibly, Shine's greatest claim to fame is that Shane MacGowan once named him as his favourite singer, in a 'Q' magazine interview. Either Shane was drunk or being over sentimental or both).