A Government decision on the 450 million euro state funds needed to build the new National Children's Hospital will not be given until September.
Plans for the facility will be submitted in a fortnight after an international expert group unanimously backed the Mater hospital in Dublin's north inner city as the best site for the facility.
Health minister James Reilly admitted funding for the hospital would be subject to a comprehensive spending review in the autumn.
A philanthropic fundraising drive to raise some of the additional 200 million euro needed will then be launched by the National Paediatric Hospital Development Board.
It is hoped the new facility - which will merge the three children's hospitals onto Eccles Street, Dublin - will be open by the end of 2016, a year behind its original target.
"It is time for everyone to come together and deliver this children's hospital in the shortest possible time," said Dr Reilly.
"The creation of a world class tertiary hospital will need the commitment of everyone, including all those involved in the delivery of health services to children and in our wider society."
The National Children's Hospital will merge the existing children's hospitals at Temple Street, Crumlin and Tallaght.
It will have about 450 beds over 16 storeys, overnight in-room beds for parents, play areas, courtyards and a therapy pool with a hydro-pool and gym. A proposed helicopter pad will now be built on top of the adjacent, lower, adult hospital.
An Ambulatory and Urgent Care Centre will remain in Tallaght Hospital to provide paediatric emergency care, elective day care and outpatient services to children on the southside.