If they allowed the domain to be deleted and only noticed now then it obviously wasn't that important to them.
If they're trying to make out that they are the "owners" then they are sorely mistaken
A .ie domain name like any other domain name is only yours to use for a fixed period of time. If you don't renew it (and you are given ample opportunity to do so in .ie) then it becomes available for anyone else to register (use)
So if your client is actively using the domain, which I assume is the case, then I'd tell the other lot politely and firmly to get lost