The Celtic mass is the liturgy of the Christian office of the Mass as it was celebrated within Celtic Rite of Celtic Christianity in the Early Middle Ages.
Sources
Two books, the Bobbio and the Stowe Missals, contain the Irish Ordinary of a daily Mass in its late Romanized form. Many of the variables are in the Bobbio book, and portions of some Masses are in the Carlsruhe and Piacenza fragments. A little, also, may be gleaned from the St. Gall fragments, the Bangor Antiphonary, and the order for the Communion of the Sick in the Books of Dimma, Mulling, and Deer. The tract in Irish at the end of the Stowe Missal and its variant in the Leabhar Breac add something more to our knowledge. The Stowe Missal gives us three somewhat differing forms, the original of the ninth century, in so far as it has not been erased, the correction by Moelcaich, and, as far as it goes, the Mass described in the Irish tract. From its size and contents it would seem to be a sort of Missale Itinerantium, with an Ordinary that might serve for most any occasion, a general Common of Saints and two Masses for special intentions (for penitents and for the dead). The addition of the Order of Baptism, not, as in the Bobbio book or in the "Missale Gothicum" ad Missale Gallicanum, as part of the Easter Eve services, but as a separate thing, and the Visitation of the Sick, points to its being intended to be a convenient portable minimum for a priest. The pieces said by the people are in several cases only indicated by beginnings and endings. The Bobbio book, on the other hand, is a complete Missal, also for a priest only, of larger size with Masses for the Holy Days through the year.
The original Stowe Mass approaches nearer to that of Bobbio than the revised form does. The result of Moelcaich's version is to produce something more than a Gelasian Canon inserted into a non-Roman Mass. It has become a mixed Mass, Gelasian, Roman, or Romano-Ambrosian for the most part, with much of a Hispano-Gallican type underlying it, and perhaps with some indigenous details. The title of the Bobbio daily Mass is Missa Romensis cottidiana, and the same title occurs before the Collect Deus qui culpa offenderis at the very end of the Missale Gothicum. This collect, which is in the Gregorian Sacramentary, occurs in both the Bobbio and the Stowe, and in the latter has before it the title, Orationes et preces missae aecclesiae romane, so that it is evident that the Roman additions or substitutions were recognised as such.
Liturgy
The Order of the daily Mass, founded on that in the Stowe Missal is as follows:
Praeparatio Sacerdotis. Confession of sins, beginning "Peccavimus, Domine, peccavimus". This and the Litany which follows are found also in the St. Gall fragments, but not in the Bobbio book. Litany of the Saints. In the original hand there are only thirteen invocations (Our Lady, ten Apostles, St. Mark, and St. Luke). Moelcaich added thirty-one more, of which twenty-four are Irish. The manuscript is wrongly bound, so that these additions look as if they were associated with the diptychs in the Canon. Oratio. The "Oratio Augustini" ("Rogo te Deus Sabaoth") is found in various ninth- and tenth-century French books (see Warren's "Celtic Church"). The Oratio Ambrosi ("Ante conspectum divinae majestatis") is inserted by Moelcaich and found in several French books. Collect. "Ascendat oratio nostra". This occurs after the Creed and Paternoster in the "Liber Hymnorum".
From the Irish tracts it seems that the chalice was prepared before the Introit, a very usual practice in both East and West in early times. It is still the Eastern practice, and is retained to this day by the Dominicans at low mass and in the Mozarabic Rite. Water was poured in first with the words "Peto (Leabhar Breac, Quaeso) te, Pater, deprecor te, Fili, obsecro te, Spiritus Sancte". The Leabhar Breac directs that a drop shall be poured at naming each person of the trinity. The wine was similarly poured on the water, with the words, "Reditit pater, indulget Filius, miseretur Spiritus Sanctus." The Introit is mentioned in the Irish tracts but not given in the ordinary or elsewhere in either missal. Probably it was sung from a psalter.