Yes, Fiolar: I forgot that I had Colman's book! IF the song was in fact published on 15 Feb 1845 (source: Oxford Companion to Irish Literature) and Colman is accurate about the date of Downing's first published poem, 10 May 1845, surely it would rule out Downing as the writer of Carrighdoun?

(According to Colman, Downing, an extremely shy girl, "singularly pretty and attractive", was romantically involved with a young man called Brennan, who got transported for political activities. She stopped writing for the "Nation" in 1847 after the departure of Mitchel: Gavan Duffy and Smith O'Brien being too moderate. Thereafter her work appeared in [Mitchel's] "United Irishman" and assumed a more radical tone. The arrest and transportation of Mitchel, and the 1848 Revolt, created such a strain that Ellen's health was broken. She then suffered intermittently from paralysis until the time of her death - though her mind was always clear. An untitled poem beginning, "Deep in the shadow of my room" records her physical and mental state shortly before her death).