The Archbishop of Dublin is defending the rule prioritising children who have been baptised in Catholic Schools.
Diarmuid Martin was responding to claims by a mother at the weekend that her four-year-old son was denied a place in a number of schools in south Dublin because he had not been baptised.
He told the Irish Independent the problem is a 'lack of places' at schools and the process of separating schools from religious orders is too slow.
Meanwhile a new campaign has been launched to prevent schools with religious patronage from turning away children who are not baptised or of that religion.
Currently, children who are not baptised remain at the bottom of some local school admissions lists - with parents obliged to send children to the only faith-based schools that will take them.
John Walshe, former special advisor to former Education Minister Ruairi Quinn, told Newstalk Breakfast changes need to be made.
"The first multi-denominational school was opened in 1978 - at this stage you think there'd be a huge growth in that number" he told Newstalk Breakfast.
"We still have fewer than 100, which is absolutely ridiculous for the modern Ireland".
"The process needs to be speeded up; the way Ruairi (Quinn) tackled it was through the divesting process - but it is very slow, it is really too slow" he added.