@ilona-miskell , assuming that what you are saying is that the property was owned by your father, then put in a revocable trust before his passing. At death of the decedent this trust became irrevocable. If that is correct then the usual "step-up" to FMV still operates. If the inheritor(s) now sell this property with the stepped-up value of FMV, then there should be NO Gain and hence no Capital Gain.
If on the other hand the property was indeed held in an irrevocable trust, then depending on exact facts and circumstances, the usual step-up may not apply. In such a case the trust may have to pay capital gain taxes on disposition of the asset. A lot depends on the actual trust document and so I would suggest that you consult a tax attorney i.e. if the prop. was indeed held in an irrevocable trust prior to passing of the decedent.