While the question assumes that irrevocability clauses are legally effective, it is sometimes suggested that even the GPL is a gratuitous licence which is revocable at will. For example, see:
Andrew's answer confirms that in the U.S., even a licence which was paid for and agreed to be irrevocable can be revoked after 35 years. However, as explained in the comments, I am not sure if 17 U.S.C. § 203 prevents revocation of a gratuitous licence within 35 years. In any case, I am more interested in the law in Commonwealth countries.
The most authoritative analysis I have been able to find is contained in Fitzgerald and Suzor, Legal issues for the use of free and open source software in government (2005) 29(2) Melbourne University Law Review 412 at 437 (footnotes omitted):
… a gratuitous licence can normally be revoked at will. This means that, in the case where one single entity controls a significant portion of the copyright in the source code for a free software package, that entity may be able to terminate the licence and users will no longer be entitled to copy or redistribute the software … In the event that a licence is revoked, it is likely that the doctrine of estoppel would prevent the copyright owner from asserting his or her rights. Equitable estoppel has been developed to prevent a person from unconscionably denying an expectation where they induce in another party (here the licensee) an assumption that a particular legal relationship exists between them, and that party subsequently acts, reasonably, in reliance upon that expectation. If a licensor releases software under a free software licence, they are essentially inviting others to perpetually use, reproduce, modify and distribute that software. If another person does in fact make use of the software, and the original licensor purports to revoke the licence (a departure clearly to that person’s detriment), the doctrine of equitable estoppel would arguably prevent the licensor from denying that the licence could not be revoked.
The authors conclude that the issue is ultimately of more theoretical than practical concern:
In practical terms, however, it would be hard for any single licensor to revoke a licence partially supporting a program — especially one which forms part of a large, distributed project … While revocation may be technically possible, it is unlikely to occur in the face of public opposition and a vigilant open source community. Regardless, as has been demonstrated over the last 12 months by the SCO Group Inc v International Business Machines Corp litigation, the developer community is more than willing to replace any code for which the licence has been revoked or that otherwise infringes copyright. For these reasons, the issue of revocability is much more a theoretical than a practical concern.