Former students at one of the country's top schools are being urged to join a campaign against "unjust State interference" in pupil admissions policy.
Education minister Jan O'Sullivan has signalled her intention to bring in new laws in the coming months to combat what she claims is inequality in school admissions.
But ex-pupils of Blackrock College, including leading figures in the law courts, business, politics, sport and entertainment, are being warned that elements of the proposals are unjust and threaten their traditions.
In a letter to members of the Blackrock College Union, known as Rockmen", union president and senior barrister Shane Murphy warns former students that the legislation would allow for the State to take control of the admission policies for all secondary schools.
"The admissions policy (at Blackrock) has engendered a positive community spirit through many generations of 'Rockmen' and has never before been so unjustly challenged by the State," he writes.
Past pupils of Blackrock College in south Dublin include former education minister Ruairi Quinn, broadcaster Ryan Tubridy, rugby star Brian O'Driscoll, Celtic Tiger financier Derek Quinlan as well as several High Court judges.
Mr Murphy says the planned new school admissions law will ban arrangements whereby brothers and sons of past pupils get priority over those who have not been to the school.
"Anything that potentially threatens the tradition where brothers and sons of past students can follow in the footsteps of their brothers and fathers through Blackrock College is a threat to that which many of us hold so dear," he writes.
Parents of pupils attending Blackrock College pay fees of up to 17,250 euro (£13,596) a year.
Ms O'Sullivan has said the proposed legislation, to be enacted before the end of the year, would ensure schools have an open-door policy to all children.
The pledge followed a report by think-tank the Economic and Social Research Institute, which highlighted that children at working-class schools were less likely to go on to university.
Ms O'Sullivan said the overhaul would get a good social mix into schools.
But in his letter to former Blackrock students, Mr Murphy says: "This legislative proposal is potentially one of the greatest challenges facing the college and the union in recent years."
He adds: "We would encourage you to become actively involved to ensure that this proposal is defeated."
Mr Murphy insists the school's admissions policy is fair and transparent "without unjust State interference".
The union is talking to past pupil organisations of other schools and is considering how best to respond to the Government's proposal, he says.
"As a union, we feel strongly that any threat to the ethos and traditions of Blackrock College should be resisted," he writes.
"A concentrated effort on the part of all past students and their families will likely be required to ensure that the current proposal is defeated."