There are about nine distinctive sets of annals that survive from medieval Ireland, each of them resulting from scribes copying older manuscripts, adding new information to them, and sometimes changing earlier entries. We know that older information (from earlier manuscripts and time periods) were tampered with for a number of reasons: words or phrases that would not have been used at the time of the events described were obviously added later; different annals which derive from a common source disagree about certain details, and those details sometimes reflect the interests of the people who changed them; etc.
It is generally accepted that all of the main surviving Irish chronicles have as their early core a text which modern scholars have decided to call the Chronicle of Ireland, which covers from about the years 431 to 720. Although no manuscript version of the Chronicle of Ireland now survives, by comparing the entries of the many annals derived from it, we can tell that it was the earliest compilation of annals created by Gaelic scribes.
The Chronicle of Ireland itself, however, was the product of at least a couple of different stages of work. The diagram below attempts to represent the layers of materials in this core text.
Stage 1: The first stage is another reconstructed text (one that doesn’t survive but whose existence has been surmised by comparison between texts) which modern scholars have called the Iona Chronicle. This is because, given the amount of attention given to people and events in Scotland and Northumbria, it is believed to have been written in the island of Iona. It may have been started as early as the 540s, but it was certainly in existence by 660. Work on it seems to have stopped c.740.
Stage 2: The manuscript of the Iona Chronicle (or a copy of it) was then taken elsewhere, probably Armagh. New entries were added up to the year 911. During this time period, the text was also edited considerably: new information was added from a “Mediterranean Miscellany” which covered the years 430 to 720 from a variety of sources, and some of the entries may have been changed subtly to promote the image of Patrick as the first apostle of the Irish.
The “Mediterranean Miscellany” included such texts as the Chronicle of Marcellinus (written in Constantinople in the early sixth century), Chronica Maiora (by the English scholar Bede), Chronica Maiora (by Isidore of Seville), Liber Pontificalis (biographies of the Popes), a chronicle by Prosper of Aquitaine, and perhaps Bede’s Ecclesiastical History of the English. This range of texts displays the wide learning of Gaelic scholars and their desire to put their history within the wider context of the history of the Christian church and Roman Empire.
Editors of annals have encoded the dates as follows: an abbreviation for the source (U = Annals of Ulster, T = Annals of Tigearnach, etc), the year, a period (“.”) and a number which increases for every entry for that year. So, for example, “U588.3” signifies the 3rd entry for the year 588 in the Annals of Ulster.