The writing of Annales Bertiniani(named after the Belgian monastery of Saint Bertin where it was written) it was written by Hincmar of Rheims and other authors.
Vikings were back and several Irish town were looted. In the next 50 years, 83 raids are mentioned in Irish chronicles.
Raid on the French Atlantic coast and from there onto Southern England.
Equipping and supplying a Viking army was a rather complicated process in the 9th century, than it was later in the period. During the earlier part of the Viking age decentralized power was unable to raise large forces without the consent of local chieftains. They were not more powerful than the hersir. Regional troops would have been erected and equipped local, the laws of the defense of Norway(Hirdskra) are an example of this.