From what I have studied so far, the Celtic Rite is now a defunct rite of the church. It had its own Missal (I actually translated it Latin/English from Latin with Gaelic rubrics) that may have dated back to St. Patrick, it’s own Breviary, Martyrology (Martyrology of Ogenius spelling???) and Liber Hymnus.
You can find fragments of it through out modern Catholic prayer books. For example, I saw a book with a prayer in it called “The prayer of St. Patrick’s breast plate”. They had their own rite of sacraments (which resembled Roman) and a way of Consecrating a church (resembled Roman I think).
I am trying to get this Rite translated and available at some point. I needed help with Latin so I am stuck at only have the Ordo translated. It is very beautiful and looks like a cross of Tridentine/Mozarabic and Byzantine.
At the Council of Trent, they allowed other Rites to exist that were older than 200 years old. Until Vatican II, most of them gave up their right to use it in exchange for the Novus Ordo i.e. the Carmelites. However, today you still have the Mozarabic Rite in Spain and the Ambrosian Rite in Northern Italy. However, the Use of York, Sarum Rite, Celtic Rite and the Use of Hereford are all but distant memories. Some priests in England though still say the Sarum Rite mass occasionally which is more ornate than the Tridentine.
With our Holy Father’s recent decrees on the old liturgies that were still around in 1962, you can use them now from what I understand also besides the old Roman Tridentine Mass. I know a priest that did the Traditional Ambrosian mass some times and a Carmelite Monk in Rochester said they are trying to learn the mass according to the Carmelite Rite.
If you are interested in looking at the Celtic Missal, let me know, I’ll post a link. The Confiteor is very beautiful, a plea to God for forgiveness followed by a litany of Saints.
The Kyrie in the Roman Rite is a shadow of that litany (some theorize), as in the Celtic Missal (here is a preview of the Mass of the Cathecumens)
Litany of saints, apostles and martyrs, virgins and the Confessions begins. O, God come to my.
And the rest.
O God, come to my assistance.
O Lord, make haste to help me.
Glory be to the Father and to the Son and to the Holy Ghost,
As it was in the beginning, is now, and ever shall be, world without end. Amen.
“Confiteor”
We have sinned, O Lord, we have sinned; forgive our sins and save us, like (how) thou guidest Noah above the waves, graciously hear us, and as you brought back Jonah from the abyss by thine word, liberate (deliver) us, who stretched out Thy hand as Peter was sinking, help us, O, Christ, son of God, thou hast did wonders with our fathers, O, Lord, and forgive our times, by Thy hand deliver us from on high.
Litany of the Saints
Christ, hear us
Christ, hear us
Christ, hear us
O Lord, have mercy
Saint Mary
Saint Peter
Saint Paul
Saint Andrew
Saint James
Saint Bartholomew
Saint Thomas
Saint Jacob
Saint Thaddeus
Saint Matthew
Saint Mark
Saint Luke
Let us pray.
All the saints pray for us,
Be merciful, spare us,
O Lord, be merciful,
O Lord, deliver us from all evil,
deliver us, O Lord
by your cross,
deliver us, O Lord
Some other resources is googling Henry Bradshaw society books for Celtic Rite, they are 1890’s books that have the ancient texts transcribed.
Hope this helps and sorry for spelling and grammar, been up for 14 hours.
Philip