This is a prayer to St. Catherine, as well as other saints and of course to God. St. Catherine of Alexandria was the patron saint of students, scholars and philosophers, so naturally she was popular with the learned poets of Ireland. Today (as with most people who ever lived) there is no historical proof of her existence or martyrdom. But according to legend, St. Catherine was a philosopher who frequented the Library of Alexandria. She was ordered by the Emperor to argue the Christian case against 50 pagan philosophers. She confounded them with her wisdom, so the angry Emperor killed the philosophers (another Irish hymn to Catherine says that God would punish the emperor for killing men of learning!), and then Catherine.
Réalta [an] chruinne Caitir Fhíona
Caitir Fhíona inghean [chasda]
Abhra uaine ag inghin Gréigríogh
Snuadh na suibhe [is] samhradh gréine*
Ar deilbh corrshúl Caitreach-fíona
Gnúis mar abhaill, ucht mar eala
Ní fhaghaim ógh ar a gruaidhse
Brighid Éireann agus Alban
Athrachta fhóireas na Luighnigh
Ciarán Caoimhfhionn, Colam Cille
An Tríonóid, mórMhuire is Míchéal
Star of the world, Catherine,
Catherine, honored daughter,
A green brow on the Greek king's daughter *
Berry's color and sun's garland*
In the shape of her curved eyes, Catherine
Face like an apple, breast like a swan,
The virgin with her cheeks will not be found
Brigid of Ireland and Scotland,
Athrachta, helper of Limerick --
Gentle white Ciaran, Columcille--
The Trinity, great Mary and Michael --
* abhra uaine/green brow: "uaine" means green or greenery. McKenna translates this as "dark brow", but I have a feeling it could also refer to the laurel crowns you sometimes see on pictures of saints (because of St. Paul's comparison of life to a race, and Heaven to the athlete's laurel). It would be especially fitting for Catherine as a Greek Egyptian to wear such a crown of victory.
* inghin Gréigríogh/the Greek king's daughter: I don't know if it means anything, but a good number of Irish fairytales have the hero meet up/quest for/marry the king of Greece's daughter. Anyway, as far as I know, European legends of St. Catherine of Alexandria do not make her a princess -- just a philosopher.
* samhradh gréine/sun's garland: sunshine.
* ó Mhac Muire/son of Mary: Jesus.
* Bhúille/the Boyle: a river
* an Tríonóid/the Trinity: the Holy Trinity
* everybody: popular Irish saints, or (Martin, Mary and the archangel Michael) saints popular in Ireland.