Complex sentence: Around the 9th century the Vikings took Moors they had captured and were using as slaves to Ireland; the Vikings spent winter months near the Shiant Isles, and Mackenzie attributes the story of the blue men to these "marooned foreign slaves".
Split into simpler sentences: Around the 9th century the Vikings took Moors they had captured and were using as slaves to Ireland. The Vikings spent winter months near the Shiant Isles, and Mackenzie attributes the story of the blue men to "marooned foreign slaves".