"My mother put the property in an irrevocable trust some 15 years later. "
There is no step-up in basis for property held in an irrevocable trust.
https://www.journalofaccountancy.com/news/2023/mar/no-basis-step-up-for-grantor-trust-assets.html
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
