 By the 9th century, the Gaels had been joined by Norse Vikings who founded towns such as Cork, Waterford and Limerick, for the most part incorporated into a maritime empire by the Dynasty of Ivar, who periodically would threaten Munster with conquest in the next century.Around this period Ossory broke away from Munster.The 10th century saw the rise of the Dalcassian clan, who had earlier annexed Thomond, north of the River Shannon to Munster.