He returned to Ireland and wrote in newspapers, but concentrated on Irish literature: he was a native speaker.
He published in 1866 (Rolls Series) the Chronicon Scotorum of Dubhaltach Mac Fhirbhisigh, a summary of Irish history up to 1150, with a glossary.
In 1875, he revised and annotated an edition of The Book of Fenagh, the house-book of St. Caillin's Abbey, County Leitrim; and in 1887 one volume of the Annals of Ulster, carrying the chronicle up to 1056.
He translated the Tripartite Life of St Patrick (1871); revised the Pedigree of the White Knight (1856); edited the text of the Poets and Poetry of Munster (Dublin, 1883); translated and added a tract on Cath Cnucha from Leabhar na h-Uidhre, and Mac Conglinne's Dream from Leabhar Breac.
[1] Elected Todd professor at the Royal Irish Academy 1882–4, Hennessy prepared a text and translation of Mesca Ulad, the drunkenness of the Ulstermen, which was published in 1889, immediately after his death; he left another old tale, Bruiden Dáderga, in proof.