I think the 6 month old Margaret who showing in 1871 is daughter to William. This looks a likely entry for her birth on IGI:
MARGARET OMAN Birth: 19 SEP 1870 Dublin
Parents: WILLIAM OMAN and MARY ANNE MURPHY
There are a couple of things you could check on original documents Paula. From
www.scotlandspeople.gov.uk, you would be able to check the marital status of son George, 18, who shows at home with parents in 1871. The transcript does not show marital status unfortunately.
Also, if you have the marriage date for William and Mary Ann/Margaret Murphy, you can confirm this date on the birth entry for the son Archibald who was born in 1874 in Paisley. From
http://search.labs.familysearch.org/recordsearch/start.html#start the date of marriage shows as 4 Fe. 1850 in Dublin.
Scottish birth certs from around 1862 include the date and place of parents' marriage. William's wife, is it Mary Ann or Margaret, quite different first names, just wondered why both come up. The marriage shows as Mary Ann Murphy and birth registrations for Margaret in Dublin and Archibald in Paisly show her as Mary Ann. I am wondering whether 'Margaret' in 1871 is a mistranscription of her name, ie., Mary Ann?
Monica