Complex sentence: Ragnall was cornered and besieged for three weeks, but without a decisive conclusion, by the High King of Ireland Niall Glúndub, while the army of Leinster which Niall summoned to join him was defeated by Sihtric.
Split into simpler sentences: Ragnall was cornered and besieged by the High King of Ireland Niall Glúndub. The army of Leinster which Niall summoned to join him was defeated by Sihtric.