You really should consult with a local tax professional (and possibly legal counsel); someone actually needs to read the trust document itself. It could very well be that the trust would continue to be considered a grantor trust (if you are the sole trustee and beneficiary and have certain powers), the fact that it is irrevocable notwithstanding.
Regardless, whoever received the refund should report it on their return since it is IRD (income in respect of a decedent). If the trust (or estate) received the refund, it can be reported on the 1041.
If the trust is not considered to be a grantor trust, then the trust actually did become a new entity upon her death (an irrevocable, nongrantor trust) and would need a new EIN.
