If the trust had been reporting under the grantor's social security number (as a grantor trust, which is the typical case) and the (now irrevocable) trust needs to file an income tax return, a EIN is needed in order to file a return (Form 1041) for the trust since it is now a separate taxable entity.
See https://www.irs.gov/businesses/small-businesses-self-employed/do-you-need-a-new-ein
