The OP had the sean-nós "Caoineadh na dTri Muire" as a point of departure. I searched the Mudcat Forum and could not find a thread that gave lyrics and/or English translation for this great song, as Mudcat does for so many other traditional Irish lyrics.

To add to the questions posed by earlier posters: the sean-nós "Caoineadh na dTri Muire" may be about women lamenting the Passion of Christ, but its singing is hardly limited to women. The late Joe Heaney disclosed that he learned it from the singing of a grandmother; that hardly stopped him from singing it himself, quite the reverse. Maybe keening itself is exclusively for women, but songs about keening?

As for the widow's keen for Art Ó Laoghaire, this text takes pride of place in more than one "from the Irish" anthology of Gaelic poems with English translations; it would only be a question of which translation was to your taste.