hayley1977:
The subscription site Roots Ireland has baptismal records in Clashmore RC records for three younger siblings of Margaret: Catherine 1816 Maurice 1822 Johanna 1825. I did not find the marriage records. Presumably the parents married before the Clashmore marriages records start in 1810.
There were no Shanahan/Shannahan records in the 1850 Griffiths Valuation head of household listing for Clashmore civil parish but there were seven Mansfield records https://www.failteromhat.com/griffiths/waterford/clashmore.htm
Roger McDonnell