Complex sentence: Their brother Alistair Og MacDonnell, had landed at Rathlin, with the final levies raised in the highlands and Islands, reputed to be nine hundred men, but in the light of Shane's decisive victory, Alistair could achieve nothing without a base on the mainland.
Split into simpler sentences: Their brother Alistair Og MacDonnell, had landed at Rathlin, with the final levies raised in the highlands and Islands, reputed to be nine hundred men. However, in the immediate aftermath of Shane's decisive victory, Alistair could achieve nothing without a base on the mainland.