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The first inhabitants in the 8th century BC Celtic tribes took about 500 years to settle in Ireland. The last ones, known as Gaels landed in the 19th cent. and divided the island into five provinces: Leinster and Meath Connaught, Ulster and Munste.
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The Vikings in 795 AD, when they robbed the prosperous monasteries. To defend themselves, the monks built cylindrical towers that served as watchtowers in case of attack. Excellent examples of these still stand throughout the country, e.g. at Glendalough, Kells.