Complex sentence: Godred was soon able to secure not only Mann, but most of the northern Hebrides and Skye; however, the Hebridean-territories won by Somerled in 1156 appear to have been retained by Somerled's descendants.
Split into simpler sentences: Godred was soon able to secure not only Mann, but most of the northern Hebrides and Skye. The Hebridean territories won by Somerled in 1156 appear to have been retained by Somerled's descendants.