Of the various sets of Irish annals, the two that have most relevance to this website are the Annals of Tigernach and the Annals of Ulster. Both are written in a mixture of Latin and Irish.
The Annals of Tigernach is the name given to the remnants of a compilation, the important parts of which – fragments covering the periods 489–766, 974–1003 and 1018–1178 – are preserved in a 14th century manuscript (Oxford, Bodleian MS Rawlinson B 488). The annals are named from Tigernach ua Braein, abbot of Clonmacnoise, who is traditionally credited with compiling the work down to his death in 1088. This attribution is, however, now thought to be groundless, but the name, being well established, remains.
Actually, the dates given to the fragments are inferred – the Annals of Tigernach does not typically employ Anno Domini dating (there are a handful, intermittently from 1024). The beginning of each successive annal is indicated by K. or Kl., standing for the Kalends (i.e. the first day) of January. Except in the central section – annals falling after 656 and before 1019 – a ‘ferial number’ is normally added to designate the day of the week on which the Kalends of January fell in that year. For instance, K.iii. means that 1st January was on a Tuesday (3rd day of the week) that year – a circumstance which will not occur again for twenty-eight years. The numerals, though, are prone to scribal error.[*] From 1019 onwards the moon’s age on 1st January (the ‘epact’, a nineteen year cycle) is added to the ferial number. For the central section there is neither ferial nor epact data. Dating entries in the Annals of Tigernach is generally achieved by comparing them with entries in the Annals of Ulster, which do have A.D. references. In fact, the earliest fragment of the Annals of Tigernach mentioned above, dated 489–766, shares a common, no longer extant, ancestor text with the Annals of Ulster.
The Annals of Ulster cover the period from 431 until 1540. Compilation began in the late-15th century, under the direction of one Cathal Óg Mac Maghnusa (who died of smallpox in 1498), and up to 1489 the manuscript (Dublin, Trinity College MS 1282) is the work of a single scribe: Ruaidhri Ó Luinín. Parts of this original manuscript are missing – some of the gaps can be filled from a 16th century copy (Oxford, Bodleian MS Rawlinson B 489).
There is, however, a well known difficulty with the A.D. sequence presented by the Annals of Ulster. Marjorie O. Anderson explains:
… something must be said about the A.D. year-numbers in AU [Annals of Ulster], which afford the usual and the only convenient method of reference. They are written in the original hand, but were certainly introduced into the text no earlier than the tenth century. The first item, the mission of Palladius to Ireland, is dated “A.D. 431”, which is the true year to be inferred from Prosper’s chronicle; and thereafter AU’s numbering is continuous. But the year sections down to the actual year 1013 are one too few. Assuming that the fault was not original, a year-section must have been lost by a copyist, perhaps somewhere in the 480s, certainly at some point before the end of the seventh century, since from about 700 onwards the addition of one to AU’s year-number usually gives the true historical date, in those cases where it can be checked. But in the sixth and seventh centuries this is so for only a minority of checkable entries.Kings and Kingship in Early Scotland (Revised Edition, 1980) Chapter 1
In the interests of simplicity, the convention of incrementing annals dated 488–1012 by one year has been followed on this website.[*] However, as Marjorie Anderson noted, pre-700 the appropriateness of this convention is not clear-cut. After analysing the contents of several sets of Irish annals, Daniel P. Mc Carthy has concluded that the Annals of Ulster’s A.D. dates are consistently one year behind 662–1012, but 457–661 the situation is rather more volatile, and he has produced look-up tables (freely available online) in which entries from the various sets of annals are displayed in synchronism on an A.D. framework.[*] Of course, whatever ‘correction’ is applied to the A.D. dates, there is no guarantee that the result will be historically correct – an event might simply have been assigned to the wrong annal.