 In 1018 , Sigtrygg plundered Kells ; he " carried off innumerable spoils and prisoners , and slew many persons in the middle of the church " . These captives would either have been ransomed or sold off into Dublin 's lucrative slave trade . When Sigtrygg raided south in 1021 , he was defeated at <unk> in County Wicklow where the new King of Leinster , Augaire mac <unk> , " made a dreadful slaughter of the foreigners " in the Kingdom of <unk> . In 1022 , the Dublin fleet sailed north against the Ulaid , only to be destroyed in a naval battle against Niall mac Eochaid , after which the Norse crews and ships were taken prisoner . 
