The Silence of Lir by Mary E. Twomey

Seeds of doubt, betrayal and rebellion begin to sprout in the Realm of the Sun and the Moon, causing division, subterfuge, and deadly attacks.
An evil that was locked up centuries ago is unleashed and let loose on the earth. There are those who fight for unity, those who struggle just to stay alive, those who battle for control, but a few begin their secret plans for a revolt.

Now they must enlist the help of one man, Finn, to help them bring the light back to the Sun. The keepers war, the North Star steals light from the Sun, and the Moon is in disrepute. The end of existence is coming, and all the while the king, the Moon, Lir, remains silent.