Ants are not like termites, and they do not eat the tree. Carpenter ants will definitely create nests in decaying trees, but they are generally not considered a great risk to the health of the tree, as they don't tunnel in the living wood. They are infamously unpopular when nibbling on the structural wood of houses, though.
In fact, ants are generally considered beneficial to the trees, since they are pretty aggressive predators of the insect world, and tend to attack and consume more damaging insects. That being said, some ant species are known to cultivate and protect aphids, somewhat to the detriment of the tree hosting them. There are also leaf-cutter ants, that can defoliate a tree and then farm the leaves below ground for the fungi that grows upon the harvested leaves.
I have never heard of these problems on an ash tree, so I think we should remove ants from the list of problems to worry about. Can we post some pictures of the specific areas of concern? Diagnosing tree problems is mighty tricky without pictures.
