An irrevocable trust can be formed in a few ways. It can be created as an irrevocable trust, it can originate as a revocable trust, or it can automatically switch to an irrevocable trust upon the passing of its originator. Regardless, once a trust becomes irrevocable, it is no longer in the hands of the individual who created it. An irrevocable trust is going to need a tax ID (also known as an employer identification number) because it can no longer use the SSN or ITIN of the individual (or individuals) who created it.
Irrevocable trusts are separate from revocable trusts because they cannot be changed (or revoked) by the individual who first donated their assets into it. The assets will stay in the trust under the initial guidelines of the trust unless both the grantor and the beneficiary come to an agreement to change it. Irrevocable trusts are most commonly created when an individual dies and tries to pass on their state. But they can also be created for the purposes of charitable giving or family purposes, as a way to ensure that the funds are definitely going to be distributed for a certain amount of time.