Hello Dana. Actually i live in tropic and i dont know much about irish moss ( is it Sagina subulata ?). Sorry to hear them died back.
I though that evergreen plant will do not do that 3 year life cycle. If they can pass 3 years without any problem they should do for another years too. Maybe its because of desease or too much period of wet feet ? You can take some photos or twigs and ask your local nursery.
