At it's roots, a trust is a contract. If it's truly irrevocable, the trust is governed by the terms of the trust and if it permits the trustee to sell any or all of the assets for the benefit of the trust/beneficiaries, the trustee may do so, regardless of what the settlor may now desire.

It's just that simple.

For me, step one would be to have the record of the title to the property checked. If it's been properly transferred to the trust, then check with the trustee to see if it can be sold. The trustee doesn't have to disclose all of the terms of the trust, but must represent that they are empowered to sell the property, etc. Naturally, use a lawyer and a good title company.