Over the past few decades, the early medieval Easter controversy has increasingly been portrayed as a conflict between the ‘Celtic’ and the ‘Roman’ churches, limiting the geographical extent of this most vibrant debate to Britain and Ireland (with the exception of the disputes caused by Columbanus’ appearance on the Continent). Both are not the case. Before c.AD 800, there was no unanimity within the ‘Roman’ cause. Two ‘Roman’ Easter reckonings existed, which could not be reconciled, one invented by Victorius of Aquitaine in AD 457, the other being the Alexandrian system as translated into Latin by Dionysius Exiguus in AD 525. The conflict between followers of Victorius and adherents of Dionysius occurred in Visigothic Spain first, reached Ireland in the second half of the 7th century, and finally dominated the intellectual debate in Francia in the 8th century. This article will focus on the Irish dimension of this controversy. It is argued that the southern Irish clergy introduced the Victorian reckoning in the AD 630s and strictly adhered to that system until the end of the 7th century. When Adomnan, the abbot of Iona, converted to Dionysius in the late AD 680s and convinced most of the northern Irish churches to follow his example, this caused considerable tension with southern Irish followers of Victorius, as is impressively witnessed by the computistical literature of the time, especially the texts produced in AD 689. From this literature, the issues debated at the time are reconstructed. This analysis has serious consequences for how we read Irish history towards the end of the 7th century; rather than bringing the formerly ‘Celtic’ northern Irish clergy in line with southern Irish ‘Roman’ practise, Adomnan added a new dimension to the conflict.