Erm, not clever at all, but Social Services as such did not exist in 1928. Poor Law authorities had their responsibilities transferred by the Local Government Act of 1929 to local councils. But a convent would not have been under the control of the local council. Did the convent also place children for adoptions? maybe the Catholic Adoption society might be able to help you.
However, someone else posted this information on RootsChat a while back. The records of Hollymount were transferred to:
The Catholic Children’s Rescue Society
390 Parrs Wood Road
Didsbury
Manchester M20 5NA
They also said that many children were placed by the Bury Guardians, presumably before 1929.
Good Luck
Nell