Vicki:
Welcome to Ireland Reaching Out!
William had at least three siblings: Ellen 1838 Margaret 1842 John 1845 The parents were married in 1837 in Knockainey and Peterswell. See transcription below from Roots Ireland
Date of Marriage:05-Feb-1837
Parish / District:KNOCKAINY & PATRICKSWELL
County:Co. Limerick
Husband ThomasSullivan
Wife JoannaHayes
I did not find Thomas Sullivan in the 1850 Griffiths Valuation in Hospital civil parish but there were 19 records with that name and possibly he was in a different parish.
I did not find any Mary Sullivan baptismal records with father William in the Hospital RC parish.
Roger McDonnell