You assumptions are correct - when the grantor passes away a revocable trust is automatically converted to irrevocable - and is treated as a separate taxing entity.
Based on your information - your mother was named as a beneficiary of that trust
and you was named as a successor beneficiary.
First of all - regarding filing requirements - see instructions - page 4 - Who Must File
The fiduciary (or one of the joint fiduciaries) must file Form 1041 for a domestic trust taxable under section 641 that has:
1. Any taxable income for the tax year,
2. Gross income of $600 or more (regardless of taxable income), or
3. A beneficiary who is a nonresident alien.
So filing was required IF there was any taxable income during the tax year.
However - if income was distributed to the beneficiary (your mother) - that income would be reported on K1 statement and taxed on her individual tax return.
Thus - there was NO additional tax liability - and only missing filing requirements.
As you realized the mistake - I suggest to requires EIN for the trust as soon as possible and start filing trust's tax return.
As there is no past tax liability you may attach a letter with the trust tax return with explanation of facts and why returns were not filed - and state that all income was included into your mother's tax returns.
I would assume the IRS will not have any concern.
But if they do - you would not have any choice - but start filing all missing tax returns.