Educate Together is taking legal advice on admissions regulations from the Department of Education which "could be discriminatory".
It says hundreds of parents, who prefer not to have their children baptised, are being blocked from accessing their schools.
Educate Together, which operates 77 schools across the State, argues that the stipulation, announced in 2011, that new schools must prioritise children from the catchment area, is denying parents who live outside the catchment area of an Educate Together school, their choice of school for their children.
In contrast, older schools – the vast majority of which are denominational – can continue to prioritise on the basis of religion. Baptised children from outside the catchment may be preferred over non-baptised children living locally.
According to Educate Together chief executive Paul Rowe, this means some parents whose children are not baptised face difficulties accessing local schools and may find it impossible to get into Educate Together schools further away, given that most are hugely oversubscribed.
Mr Rowe said it had been hoped the 2015 Admissions To School Bill would address the issue of religious schools, which are publicly funded, being allowed to prefer baptised children.
Minister for Education Jan O’Sullivan has said the legislation was drafted in the context of the Constitution and equality legislation. “We can’t within that take away the provision whereby schools can protect their own ethos.”
Educate Together schools
Educate Together schools have become increasingly oversubscribed, with hundreds of parent from outside school catchment areas applying for junior infants places, particularly in Dublin.
“Some of the schools are receiving four, five or six times more applications than available places,” said Mr Rowe.
For example, Balbriggan ET national school has 258 applications for 48 places. Holywell national school in in Swords, Co Dublin has 335 applications for 90 places, while Castleknock ET national school has 350 applications for 58 places.
Mr Rowe says the restrictions on new schools, compelling them to prefer children from the local area, may contradict powers granted to boards of management over this area under the Education Act.
“Educate Together has, like other patron bodies, sought legal advice on the issue,” he said. “The fundamental underlying issue however, is that many more Educate Together schools are needed."
It could also be necessary for Ireland to comply with its international legal obligations.
A spokeswoman from the department said: “The reason for establishing new schools is to serve a local community where a local demand exists and hence priority is given to pupils from the local area."
The United Nations has repeatedly called on Ireland to end religious discrimination in publicly-funded schools.
Most recently this week, the UN Council on Economic, Social and Cultural Rights in Geneva called on Ireland to “take all necessary measures to bring all relevant laws, including the Equal Status Acts 2001 and the Education (Admission to Schools) Bill 2015, in line with the international human rights standards and to increase the number of non-denominational schools at the primary and post-primary education levels”.