The reported side effects
of acyclovir include hallucinations, psychosis, lethargy, mood changes, and confusion. However, most of these occur in much higher doses than yours. Also, I stress "reported"; cause and effect is unclear. The specific words you use -- mood change, irritability, and depression (which is what your sympoms, as a group, suggest -- as you probably know) have not been described, but I suppose it is possible.