Anna,
Margaret’s birth is probably before 1864 in which case there won’t be a birth certificate. You might find her baptism in church records but they are unlikely to be on-line. She appears to have been born into a Presbyterian family so you would need to search all Presbyterian records around Ballyclare in PRONI.
I don’t see a marriage for them in Ireland, nor do I see Thomas in the 1901 census though there is a TM of the right age, marital status and religious denomination in Belfast workhouse then. You would need to check workhouse admission records in PRONI to see if it’s the same man. Occupation is labourer rather than flax buyer so it might not be the right man. He may have been in the infirmary. Can’t see a death in Belfast for him before April 1911 (which is when the 1911 census was taken). The name is pretty common and he might have died elsewhere in Ireland. 26 Thomas Montgomery deaths 1901 – 1911. You could check through them. You’ll be able to eliminate some because of age (eg children). Shouldn’t take too long. There's a 1911 death on an Ancestry tree which I assume is yours but it doesn’t look right. He was a carpenter from Maryville St and was still alive at the time of the 1911 census.
http://www.census.nationalarchives.ie/pages/1911/Antrim/Windsor_Ward/Ma…
Margaret is shown as dying in December 1919. I don’t see a death in the records that matches that.
Do you know where any of the family are buried? Possibly Thomas is in a family plot.
