Libby,
Margaret Irvine’s birth was on 23.7.1886 (under the spelling Ervine, a common variant). Father was William John Ervine and mother was Ann Jane Stevenson. Birth occurred in Longstone St, Lisburn.
https://civilrecords.irishgenealogy.ie
William John & Ann Jane appear to have married in Lisburn in 1881. Marriage in 1881 aren't on-line free yet. You can view the original certificate on-line on the GRONI website, using the “search registrations” option:
You will need to open an account and buy some credits. It costs £2.50 (sterling) to a view a certificate. Ann Jane’s father appears to be James but you should get the certificate to confirm that.
Ann Jane and her potential sister Elizabeth were probably born before the start of statutory registration in Ireland (1864) and so you would need to search church baptism records to confirm both had the same parents. Though perhaps the addresses and father’s occupations on the marriage certificates will indicate whether they are the same.
Tradition was to marry in the bride’s church which should be on the marriage certificate (unless it was a Registry Office marriage). That church may be the place to look for her baptism and that of any siblings.