Molly,
There are 2 townlands in Co Antrim called Drumadarragh. One is in the parish of Killagan (not too far from Ballymoney) and the other is about 25 miles south in the parish of Kilbride, near Ballyclare. Both townlands were quite small with less than 20 houses in the 1901 census.
Statutory registration of births & deaths started in 1864, so if you are looking for a birth certificate for Jessie there isn’t one to be found. You might find her baptism but not all churches have records for that period and of those that do, not all are on-line. Tradition was to marry in the bride’s church, so if David & Jessie married in a Presbyterian church, that might indicate she at least, was a Presbyterian. (Looking at the 1901 census for the Ballymoney area, most of the Horners listed there were Presbyterian, so that fits. So they will have originated in Scotland, probably in the 1600s, as is the case for the majority of the population in that part of the county).
I searched the death records for John & Rachel Horner. If either died before 1864 then there will be no record. I did not see a John Horner death that fitted perfectly though there were one or two possibles. This one died in Rosedermot which is only a couple of miles from Drumadarragh:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1884/06328/4814660.pdf
He was a widower and the informant was his daughter Margaret Horner of the same townland.
There was only 1 Rachel Horner death though. She died 10th Aug 1870 aged 68, registered in Ballymoney. You might want to look at it. If it’s the correct Rachel, then it should say whether her husband was still alive, which may help. Also his occupation. Deaths 1864 – 1870 are not on-line free yet. You can view the original certificate on-line on the GRONI website, using the “search registrations” option:
https://geni.nidirect.gov.uk
You will need to open an account and buy some credits. It costs £2.50 (sterling) to a view a certificate.
I searched Griffiths Valuation for both Drumadarraghs c 1861 but did not see a Horner family in either. However if they were labourers then they often moved to follow the available work and can be hard to trace. (I can see that Jessie gave her father’s occupation as farmer when she married, but that is not borne out by any records in Ireland. The tithes don’t list a single John Horner farming anywhere in Antrim c 1830. Possibly she may have promoted him a little when giving his occupation? That wasn’t unknown).
If Jessie was born in Drumadarragh in Killagan and was Presbyterian then she may have been baptised in Kilraghts, Finvoy, Loughguile, Clough or Rasharkin. Presbyterians in Ireland don’t use the parish system. Which church you attend is a matter of choice, and not necessarily the closest to where you live. So you often have to try several churches before you find the correct one. The records are not on-line but there are copies in PRONI, the public record office in Belfast. You could get a researcher to search them for you. Researchers in the PRONI area: http://sgni.net
There were a couple of McCollum farms in Ballynaloob, Killagan in the tithe applotment records for 1828 (it’s not far from Drumadarragh), so possibly one of those was Rachels’ family. No Horners listed in the parish which makes me think they were labourers.
http://www.irishgenealogyhub.com/antrim/tithe-applotments/killagan-parish.php
Drumaheglis to Drumadarragh in Killagan is probably about 10 miles, so if Jessie came from Killagan then they might have known each other in Ireland. If she came from the one in Kilbride parish, then I doubt they would have had the chance to meet.
There are not a lot of records for that area in the mid 1800s. You might just find some school attendance records for Jessie if she attended Cloughmills school. Their records are in PRONI in a paper file SCH/1174. I don’t know when they start though. Otherwise, apart from a baptism record, I doubt there will be any other record for Jessie in Ireland, if she left aged 20 and still single. (She would have been in the 1851 and 1861 censuses but they are both destroyed).
Elwyn