As far as the Irrevocable Trust is concerned, if all of the assets
have been distributed, then you can prepare
a final return for that using the final distribution
as the end of the period. If there is a small amount left in the trust account to pay any winding up expenses, that's OK too & you don't need to hold off filing
the final return for that; just estimate what the final expenses will be and include them (your fee ?) in the final return as if they have been paid.
As far as an income
tax return for the estate, that will depend upon when the estate is settled; depending upon the amount of the assets in the decedent's own name, if any, the estate may be required to stay open for up to 1 year, by law
You should check with the attorney handling the estate as to when the probate process, if any, will be completed. If there are income producing probate assets, the income from these assets would be what is reportable on a fiduciary return for the estate.
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