There are several ways in which an offer can be terminated. The first way to terminate an irrevocable offer is for the offerree to reject the offer, either by denying it or by presenting a counter-offer. The second way is for a period of time laid out in the original offer to expire. If the offer states that it must be accepted by a certain time, but it is not, then the offer to provide agreement to the contract is considered terminated.
An irrevocable offer, or any other offer for that matter, can also become unenforceable in several other circumstances. If the party which offered the contract dies or becomes legally barred from entering into a contract, for instance due to mental incompetence, it will be deemed unenforceable. A contract can become void if a law is passed that would cause the contract to be contrary to statute.