Peter,
It’s difficult to know what record there would be of an unmarried girl getting pregnant in the 1860s. It happened all the time, and the church baptism records contain many illegitimate births. I see from the 1901 census that the family was Presbyterian. They presumably attended Bellaghy Presbyterian church. The Kirk Session would probably have reprimanded her if she was known to be pregnant outside marriage, and would have asked her to name the father (so he could be held liable for supporting the child). If the Kirk Session minutes have survived then it might be mentioned there. They don’t appear in the records for that church held in PRONI. You could write to the Minister to see if any exist, but I am not optimistic. But otherwise I don’t see a girl getting pregnant as being terribly noteworthy (outside her immediate family) and don’t think it would be recorded anywhere.
http://www.census.nationalarchives.ie/pages/1901/Londonderry/Bellaghy/B…
I searched the passenger lists on Ancestry for Mary’s journey to Australia in the 1860s but did not find it. (She is unlikely to have sailed straight from Ireland as there weren’t many direct sailings. She’s more likely to have gone to London or Plymouth and joined a vessel there).
I noted a couple of probate abstracts relating to the family:
Letters of Administration of the personal estate of Andrew Seawright late of Ballymacombs County Londonderry Farmer who died 9 April 1878 at same place were granted at Londonderry to Margaret Seawright of Ballymacombs the Widow.
Seawright Martin of Ballymacombs county Londonderry farmer died 17 June 1922 Probate Belfast 23 January to Andrew Adams and John Junkin farmers. Effects £36.
The 1878 file was destroyed in the 1922 fire in Dublin and all that exists is the abstract. The 1922 will is on-line on the PRONI wills site.
https://apps.proni.gov.uk/WillsCalendar
Looking at Griffiths Valuation for 1859 the family homes are shown in Ballymacombs More. It was plot 25 which was a 111 acre farm jointly farmed by 5 people, 3 of whom were named Searight (John, Martin & Andrew). The farm was at Sewright Hill (that’s how it was spelled) which today is up a lane off the Newferry Rd.
There were Smyths also living nearby, and they too were Presbyterian. Possibly that was Ellen Rose’s family.
NB You have Ellen Rose Smyth as born “Down Patrick, Ballyscullion, Down, Ireland.” I think that’s a bit mixed up. Ballyscullion is in Co. Derry/Londonderry. Downpatrick is a town about 50 miles away, in Co. Down. Farmers tended to marry locally so I suspect the Smyth family are also from Ballymacombs More, in Co. Derry. Especially as there is a family of that name listed in Griffiths.
http://www.askaboutireland.ie/griffith-valuation/index.xml
Incidentally, being Presbyterian indicates the Seawright & Smyth families are probably of Scottish origins. They probably settled in Co. Derry in the 1600s as part of the Plantation of Ulster. Up to 200,000 Scots moved to Ireland then, representing something like 15% of the entire population of Scotland.
