 Dublin enjoyed a sustained period of peace while Sigtrygg 's men served in the armies of Brian . Sigtrygg never forgot the Ulaid 's refusal of aid when he fled from Dublin , and in 1002 he had his revenge when his soldiers served in Brian 's campaign against the Ulaid and ravaged their lands . His fleet raided Ulster , and he plundered Kilclief and Inis Cumhscraigh , taking many prisoners from both . They served under Brian against the Ulaid again in 1005 , and against the Northern Uí Néill in 1006 and 1007 . Cenél Conaill , the last of the Northern Uí Néill Kingdoms , submitted in 1011 , and Brian was formally recognised as High King throughout Ireland . 
