She will leave the board of the Irish National Stud (INS) at the end of the year after a period of unprecedented turmoil.
She had remained in the €12,600-a-year post despite a partial clear-out of the board by a former agriculture minister, Brendan Smith.
This followed a series of costly legal battles and embarrassing controversies surrounding the running of the company.
Some of this focused on Ms O’Reilly’s unsuccessful efforts to resolve the acrimonious fallout from an affair between two of its most senior employees.
This ended with both parties, former CEO John Clarke and former stallion nominations manager Julie Lynch, taking High Court cases against the INS. The costs of these are believed to have hit €1m.
The company was ultimately criticised for its handling of the affair and for not keeping its parent department informed of the damage it was doing.
Other issues involved the level of un-tendered business which the INS was doing with companies associated with members of the board.
In Ms O’Reilly’s case, the INS bought €93,646 worth of goods from Waterford Crystal over a five-year period. Her husband, Tony O’Reilly, and her brother, had controlled Waterford Wedgwood.
At a Dáil committee, the INS was also forced to defend sizeable profits key directors made by buying into stallions controlled by the company they were appointed to run.
In 2007, Ms O’Reilly, and two other board members shared in a €2.9m windfall when a record-breaking stallion controlled by the INS was the focus of a once-off investment that valued it at €45m.
This horse, Invincible Spirit, was part-owned by Ms O’Reilly and she chaired the subcommittee that managed it on behalf of her fellow investors. She has continued to profit from this syndicate.
Separately, the company was forced to ask the Department of Agriculture for more financial support after incurring significant losses and exhausting its borrowings in 2009.
However, at about the same time, Ms O’Reilly felt compelled to write a detailed letter defending the expenses regime she authorised.
Ms O’Reilly joined the INS in 1991 and was appointed chairman in 1998.
Internationally, she is an award-winning breeder and has been credited with helping improve the stud strategies of the company.
She was present to welcome fellow horse racing enthusiast, and INS customer, Queen Elizabeth, to the Tully farm last year.
However, cleaning up the horses for the queen’s visit caused the death of one stallion, Verglas, who was originally owned by Ms O’Reilly and in which she had retained a shareholding.