Complex sentence: By the tenth century Norse influence had grown, and Arran formed part of Sudreys or "Súðreyjar", administered either from Dublin or the Orkneys (Nordreys or "Norðreyjar") and nominally under the control of The King of Norway.
Split into simpler sentences: By the tenth century Norse influence had grown, and Arran formed part of the'Sudreys', Old Norse- "Súðreyjar". Administered either from Dublin or the Orkneys, (the Nordreys- "Norðreyjar") and nominally under the control of The King of Norway.