The history of the so-called “Fragmentary Annals of Ireland” is not entirely clear; a copy of the manuscript was made in the 15th century, which gives an end date to its compilation. One of the earlier scribes took a great interest in the Vikings and it is those items which are included below.
The following is an extract from the translated text at the Corpus of Electronic Texts Edition archive.
FA703: In this year the men of Ireland accepted a single regulation and rule from Adamnán regarding the celebration of Easter on Sunday, the fourteenth of the moon of April, and regarding the wearing of Peter’s tonsure by all the clergy of Ireland; for there had been great disturbance in Ireland until then: many of the Irish clergy were celebrating Easter on Sunday, the fourteenth of the moon of April, and were wearing the tonsure of Peter the Apostle, following Patrick. Many others, however, were following Colum Cille, celebrating Easter on the fourteenth of the moon of April no matter on which day of the week the fourteenth happened to fall, and wearing the tonsure of Simon Magus. A third group was not in accord with either the followers of Patrick or those of Colum Cille. So the clergy of Ireland used to hold many synods. And this is how those clerics used to come to the synods: with their people, so that there used to be battle challenges, and many slain among them; and many evils came to Ireland on that account, i.e. the great cattle plague, and the vast famine, and many plagues, and foreigners destroying Ireland. It was like that for a long time, that is, until the time of Adamnán. He was the ninth abbot of Iona after Colum Cille.
FA849.2: Indrechtach, the abbot of Iona, came to Ireland with the holy relics of Colum Cille.
FA849.3: Also in this year, i.e. the 6th year of the reign of Mael Sechnaill, Amlaíb Conung [Óláfr konungr], son of the king of Lochlainn, came to Ireland, and he brought with him a proclamation imposing many tributes and taxes from his father, and he left suddenly. Then his younger brother Ímar [Ívarr], came after him to levy the same tribute.
FA852.1: In this year, many [Irish] abandoned their Christian baptism and joined the [Norwegian] Vikings, plundering Armagh, and stealing its riches. But some of them did penance and made atonement. FA856: Áed, the king of Aileach [in Ireland], the greatest king of his time, made war on the fleet of the Gall-Gaedil: they are Gaels and foster-children of the Vikings, and sometimes they are even called Vikings. Áed defeated them and slaughtered the Gall-Gaedil, and Áed brought many heads away with him; and the Gaels were entitled to do that killing, for [the Gall-Gaedil] did just as the Vikings did.
FA858: Although Mael Sechnaill did not make this expedition to take the kingship of Munster for himself, it was worth coming to kill what he killed of Gall-Gaedil there, for these were people who had forsaken their baptism, and they were called Vikings because they behaved like Vikings and they had been fostered by them; and though the real Vikings were evil towards the churches, these were much worse wherever they were in Ireland.
FA871: Amlaíb was sent by his father Gofraidh from Ireland to Lochlainn to help him fight the Vikings, for the the Vikings were warring against him.
FA914.1: A great fleet of Vikings landed at Port Láirge [in Ireland] and plundered northern Osraige and stole much wealth and livestock with their ships.