The Vikings first attacked Ireland in the 790s, and the earliest raids and settlements were in the north and east of Ireland. The raids seem to have been most severe in the 830s and early 840s, when the Vikings established a number of permanent coastal settlements ( Dublin, Waterford, Limerick, and others). Thereafter the number of recorded raids diminishes, and Irish kings are accredited with a number of victories against the foreigners. The Norwegian Vikings in Ireland were further weakened by the arrival of a large Danish fleet in the late 840s, which initiated a period of civil war, which included a great sea-battle in Strangford Lough in which several hundred ships were involved. A Norwegian fleet came to restore control of the Scottish Isles and Ireland. The ambitions of the Dublin kings were thereafter directed more towards Scotland or, in the tenth century, to the Viking kingdom of York, than towards the interior of Ireland. From the 850s on the Viking coastal settlements were no real threat to the Irish kingdoms, save that they were used by various Irish kings as allies in their own internal wars. The short period of intensive raids disrupted monastic life and monastic workshops, destroyed countless irreplaceable libraries, and sent scholars and books overseas to the Carolingian court; in the long term the Viking presence may have moved the cultural and political strength of Ireland from the coastal regions to the interior, and, by the introduction of trading towns and, later, coinage, may have had a vitalizing effect on economic life.
Similar problems occur when trying to assess the impact of the Vikings in Francia. Were they anything more than an irritant, a complication in an already complex political picture, a potential ally for a discontented count, or an occasion for him to build up a local reputation as a defender of his county? They were all those things. But it is also clear that the numbers involved in raiding in Francia were far larger than in Ireland, partly because the potential profits were so much larger, and the periods of raiding were prolonged and intensive. Attacks on towns and insecurity in the countryside put a premature end to the commercial boom of Charlemagne's reign. There must have been considerable depopulation in coastal districts,
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