Thanks. I wrote to Barnardo, who didn't have any info but kindly forwarded my email to a relevant authority.
They replied with this. It might be useful to others ... I am about to write to them to see where it leads.
You mention that your great-aunts were born in Barnacullia, which we believe is near Glencullen in Co. Dublin, and were in St. Joseph's.
The Daughters of Heart of Mary ran St. Joseph's, Tivoli Road, Dun Laoghaire. This could be the school your great aunts were sent to.
You should write to The Archivist, St. Joseph's, Tivoli Road, Dun Laoghaire. Tel: (01) 2801204.
As it is possible that your great-aunts were adopted/fostered, or what was known then as boarded out, it is likely that the then Dublin Boarded Assistance was involved in their care. The records are held by: Tusla Child & Family Agency, Information & Tracing Service, 2nd Floor, Units 4/5 Nexus Building, Banchardstown, Corporate Park, Ballycoolin, Dublin 15. Tel: (01) 8976881.
Everyone has been really helpful.