Complex sentence: The alliance was formalised by a remarkable marriage between Arnulf and one of Muirchertach's daughters, recorded by Orderic and "Brut y Tywysogion", and also alluded to in the "Annals of Inisfallen".
Split into simpler sentences: The alliance was formalised by a marriage between Arnulf and one of Muirchertach's daughters. The remarkable alliance between Arnulf and the Irish is recorded by, not only Orderic, but in "Brut y Tywysogion" and alluded to in the "Annals of Inisfallen".