I wrote to the Federal Benefits Unit at the US embassy in Dublin and asked the question - and got a reply! good news:
There will be no other deduction to your US Social Security benefits when you reach your US full retirement age, even if you become entitled to an Irish State Pension, due to the Fairness Act.
Sincerely yours,
Federal Benefits Unit (SMC)
Embassy of the United States of America
42 Elgin Road, Ballsbridge, Dublin 4, Ireland
So it does seem that the repeal of that Act will make a difference to those getting two pensions.