Substantial annalistic chronicles of Irish affairs exist in a number of medieval versions, but they exhibit considerable variation both in the sequences of events and the chronological apparatus used to link each year to the Julian calendar. Of these, the Anno Domini years of the Annals of Ulster have been principally relied upon by historians. However, these are demonstrably incorrect from the seventh to the eleventh centuries. Moreover, its remaining chronological data of ferials and lunar epacts at the kalends of January, that is, the day of the week and the age of the moon on 1 January, are almost all interpolations by a later scribe. On the other hand, the Annals of Tigernach and the Chronicum Scotorum have only kalends and ferials marking the commencement of each year from the Incarnation up until the mid-seventh century. Because these kalends and ferials are susceptible to scribal miscopying they were dismissed by historians and textual scholars as “hopelessly confused”. However, analysis of the 28 year cycle of the ferials reveals that they possess a powerful error-correction property. Exploitation of this property has enabled the restoration of all the missing kalends and erroneous ferials of the Annals of Tigernach and Chronicum Scotorum, as well as of the closely related Annals of Roscrea, known collectively as the Clonmacnoise group. Using computer table structures, the kalends and ferials and events of these three have been synchronized with the Anno Domini years over the range AD 1–1178, and this tabulation, with cross-references to the other Irish medieval annals, has been made available online at www.irish-annals.cs.tcd.ie. In this chapter the process of analysis, correction, and synchronization is illustrated, taking the year of the death of St Patrick as an example.