Answer
With King Muirchertach Ua Briain, Magnus formed a significant alliance in Ireland. This alliance was crucial for Magnus's success in establishing his power in the region. Muirchertach was a powerful Irish king who controlled a vast territory in the east of the country. By allying with him, Magnus gained access to Muirchertach's resources and support, which enabled him to consolidate his position in Ireland and expand his influence.