Complex sentence: Sigtrygg didn't forget the Ulaid's refusal, and in 1001 his fleet plundered Inis Cumhscraigh and Cill-cleithe in Dál Fiatach, taking many prisoners, with his forces also serving in Brian's campaigns against the Ulaid in 1002 and 1005.
Split into simpler sentences: Sigtrygg didn't forget the Ulaid's refusal, and in 1001 his fleet plundered Inis Cumhscraigh and Cill-cleithe in Dál Fiatach, taking many prisoners. Sigtrygg's forces also served in Brian's campaigns against the Ulaid in 1002 and 1005.