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In page Brian Boru:

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Then in 993, Brian, now in control of much of Munster and gaining ground in Leinster, and unable to make significant progress against the High King on land[citation needed], decided to utilise his naval superiority to attack Mael Sechnaill. His fleet sailed up the Shannon and invaded the Kingdom of Breifne, in what is now counties Leitrim and Cavan. In doing this, he put pressure on the High King as he was now open to attack from both north and south.[3]