There is a warning that parents may have to pay to hold on to places for their children until the free preschool year kicks in for them.
Early Childhood Ireland says plans to allow parents enrol children at three times during the year are "completely impractical".
Budget 2016 gives parents the chance to start children off in preschool in September, January or April.
However, CEO of Early Childhood Ireland Teresa Heeney says preschools need to be full from September each year.
Ms Heeney said: "What that mean for parents is that they are not going to be able to get a place in the setting that they want from September.
"They may have to choose to go to another place which maybe is not what they might prefer to do.
"Alternatively, families with the means may decide to pay from September to hold a setting until January, until their preschool year kicks in."