Since an irrevocable trust can never be changed, there really is no need for witnesses. As long as the trust satisfies the grantor's wishes, and both the trustee and grantor sign the document, the irrevocable trust is complete and official.
An Irrevocable Trust Needs Two Witnesses
Due to the nature of an irrevocable trust, at least two witnesses should be present at the time it is made. Since an irrevocable trust means that the terms can never be changed or altered, trustees cannot add or delete any beneficiaries, and you can only remove the trustee if he dies or agrees to resign, you will need witnesses to be sure the transaction is not coerced upon an unwilling grantor.
You need at least two witnesses for an irrevocable trust. Witnesses can ensure that the grantor is creating the irrevocable trust of his own free will. Witnesses can also testify in probate court and help to ensure that the grantor's wishes are carried out.