He wrote nine operas, four cantatas, numerous piano pieces and songs as well as a limited amount of chamber music.
As a foreign national he was not allowed to attend the Paris Conservatoire, instead he continued his education on the piano with George Alexander Osborne (1806–1893) (before 1844) and Frédéric Kalkbrenner (1785–1849) (mid-1840s) and in composition with Victor Dourlen (1780–1864) and Fromental Halévy (1799–1862).
In these considerably more ambitious pieces he dispenses with a strophic structure, employs more dramatic development and some technically advanced piano writing.
[4] Of his four cantatas, the first, Paraguassú (1855, Théâtre Lyrique), was the most extended piece, dealing with a 16th-century episode from Brazil, for which he was awarded with the Knighthood of the Order of the Rose by the Brazilian emperor Don Pedro II.
[7] Yet he clearly identified himself as Irish: the change of name in 1859 is one proof for it, but he was also a member of the 'Anciens Irlandais' community of Irishmen in France and composed works relating to Ireland such as the Mac-Mahon Marche op.
[9] Joseph O'Kelly died of bowel cancer on 9 January 1885 in Paris, in the apartment of his brother George in the 17th arrondissement.