The family had hoped that the estate would enter public ownership
It has been conformed that Westport House, which has been owned by members of the Browne family for nearly 400 years, is to be placed on the market today with a guide price of €10m.
The family are understood to be reluctantly selling the property, The Irish Times reports that it carries debts of close to €1m - the owners had hoped that the State would take the estate into public ownership, but no agreement was reached between Mayo County Council and NAMA.
The 13 bedroom house sits on a 455 acre estate, it is a tourist attraction and features Georgian rooms and gardens and a Pirate Adventure Park.
During the Tiger years, a large portion of the estate was used as security on a loan of €6.5m to develop the estate.
This plan failed to materialised and the property was acquired by NAMA.
Loans associated with Westport House were due to be sold as part of NAMA's Project Arrow portfolio - but were withdrawn from the sale due to concerns from locals in Mayo.
The family issued a statement relating to the decision to sell: "There has been no meaningful progress made over the last four months since the future of Westport House became the subject of local concern and widespread media coverage.
"The family's hopes were raised last September when it was said that Mayo County Council were to step in and, as a first step, would acquire the loans from NAMA.
"The Browne family has regretfully accepted that as a result of this inaction there is now no choice but to put the entire property at Westport House up for sale."
The delay had been stalled as the family hoped that the estate would be brought into public ownership.