From the time of the earliest surviving Irish psalters, they have an equal three-fold division using the Vulgate translation and numbering system.
- Book I: psalms 1-50
- Book II: psalms 51-100
- Book III: psalms 101-150
These books would be marked in psalters with an ornate capital letter for psalm 1, 51, and 101. There are references to early medieval people saying their ’50s’ or requesting someone to say a ’50’ on their behalf (sometimes in wills where it would be for the repose of their soul).
The origins of the three-fold division are unresolved. McNamara has suggested that the three-fold division may have been known to Hilary and Augustine of Hippo. He believes that Hilary only knew of three-fold divided psalters based on his Prologue to the Psalms. He also asserts there is evidence that it was known to Augustine and Cassiodorus. If these church fathers knew of the three-fold divisions of the psalter then it appears to have died out after them. The Irish appear to have been its greatest proponents. Given the impact that is found in Anglo-Saxon England, it appears that these divisions appeared very early. The Vespasian Psalter, probably compiled at Canterbury in c. 730 CE, uses the three-fold divisions as does the Salaberga Psalter (Northumbrian, early 8th century) and other southumbrian psalters from the late 9th to early 10th century. However, I think some caution is warranted as the oldest Irish psalter, the ‘Cathach of St Columba’ and the Old Irish Treatise on the psalms, do not have or mention the three fold division. Yet, the Irish Teaching Bible, used for seminary training in c. 800, uses the trifold division and McNamara believes it is assumed in the Old Irish Treatise. Still yet, a psalter from Canterbury seems to be among the earliest opening up other questions.
The three-fold division is also found in the breviate psalter of Æthelwald in the Book of Cerne, produced about c. 825 CE. In this breviate psalter, the purpose of dividing the psalter into three equal divisions is lost because the abbreviated psalter omits some psalms completely, meaning that none of the three ‘books’ of psalms in Cerne actually has 50 psalms. So by c. 825, the divisions had become so standard their original meaning — whatever was behind that meaning — had been lost.
Some of you might be thinking that the Celts (Irish and Welsh) had a fondness for three-fold divisions, particularly shown in the Welsh Triads and later Irish Triads. These triads were mnemonic devices used by bards (poets) to remember stories on similar topics, such as three most generous kings or three greatest pack horses, etc. Each of these snippets of legend in the triad was intended to remind the poet of an entire story. Groupings in three do abound all over the place in early medieval writings. There are also triads within Bede’s Ecclesiastical History of the English people, such as Aidan of Lindisfarne’s three miracles worthy of memory. Sometimes Bede’s triads are grouped together, such as Aidan’s miracles, or other times spread out across the History, such as three stilling of the stormy sea miracles (Germanus of Auxerre, Aidan by proxy through Utta, and Œthelwald of Farne). So it is true that early medieval peoples, Celts in particular, but also perhaps the English (as shown by Bede), had a fondness for traids, but recall that all these sources survive in Christian texts by Christian writers, even if they record pre-Christian traditions. It is quite possible that the trinity, preached fervently as early as St Patrick in Ireland was the inspiration.
Martin McNamara (2000) The Psalms in the Early Irish Church. (Collected studies). Journal for the Study of the Old Testament Supplement series 165.
Michelle P Brown (1996) The Book of Cerne: Prayer, Patronage, and Power in Ninth Century England. British Library and University of Toronto Press.