Complex sentence: Somerled became aware of the growing army at Renfrew and decided to preemptively strike against the gathering forces, assembling an army of 15,000 from all parts of his kingdom that he then had carried to the shores of Firth of Clyde, in an armada of 160 birlinns.
Split into simpler sentences: Somerled became aware of the army gathering at Renfrew and decided to make a preemptive strike against it, assembling an army of 15,000 from all parts of his kingdom. He then had his army carried to the shores of Firth of Clyde in an armada of 160 birlinns.