The 19th century saw many songs, screeds, books, and articles about Ireland and its struggles, and its early history and legends, much romanticized and treated as fact rather than legend.
The "Bard of Armagh" seems to be of that time; its language is of that time.
Any relationship to the time of Donnelly is dubious.

An interesting book for young readers, published in 1886, was one of many keeping the old stories alive.
"Young People's History of Ireland," by George Makepeace Towle, one of the 'romantic' historians of the time, published in Boston.

From his Introduction-
To justify her oppression, England has resorted to a system of misrepresentation and misreport. Irish antiquities have been doubted and belittled. --- The ancient history of Ireland has been set down as unreliable, mythical, - a story born of Celtic pride, imagination and passion." .....................
"Yet the student who turns to the history of Ireland finds at a glance that he has entered an original and authentic region, on a study not only national but racial. ......... of music, coming down from pre-historic times, and still sung by peasant girls and played by the wandering minstrels; ......."

The history and legends of Scotland were treated in much the same way.