David,
It would not be surprising if the Greer family was Presbyterian. The area around Ballymena is dominated by Presbyterians and Greer is a common Scottish surname so it’s likely they’d be of that denomination. Houston & Kyle are also common names around Ballymena (again of Scottish origins) so it wouldn’t be too surprising if both 2 brothers knew their wives in Ireland.
And certainly John did. I found a marriage for John Greer and Peggy Houston at Ballymena Registry Office on 28.9.1850. (Peggy is a diminutive for Margaret). 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.
John was a weaver from Limnaharry, Ahoghill, aged 21 and his father was Joseph Greer a farmer. Peggy was from Slievenagh, also near Ahoghill, aged 23 and her father was James Houston, farmer. Witnesses were William Barr & Hugh Kirkpatrick. Limnaharry & Slievenagh are fairly close together just south of Portglenone village.
I looked for Joseph Greer in Limnaharry in Griffiths Valuation (c 1860) and also Robert Houston in Slievenagh. I did not find either listed in those townlands. The tithe applotment records for 1825 (which should list most farmers) have a Robert Houston in Craignageeragh which is close by. Perhaps that’s Margaret’s father.
http://www.irishgenealogyhub.com/antrim/tithe-applotments/portglenone-parish.php
If the families were both Presbyterian and lived around Ahoghill and Portglenone, then there are at least 6 churches they might have attended. Ahoghill has 3, as did Portglenone. Ahoghill 1st records start in 1841, the 2nd in 1835 and the 3rds in 1859. Portglenone 1sts start in 1826, 2nds in 1821 and the 3rds in 1869. There are copies of all those records in PRONI (the public record office) in Belfast but they are not on-line. A personal visit is required to view them.
The fact that they married in a Register Office is interesting. That might indicate they were Brethren (sometimes called Plymouth Brethren though in Ireland they don’t favour that description). Quite a common denomination around Ballymena. Often comprising people who used to be Presbyterian. If so, you may not find any church records for them since that denomination don’t put their records in the public domain. But your couple may have had other reasons for marrying in a Register Office.
Probate abstract from the PRONI wills site:
The Will of James Houston late of Slavenagh County Antrim Farmer deceased who died 15 May 1878 at same place was proved at Belfast by the oaths of William Logan of Limnaharry and John M'Kay of Ballybeg both in (Ahoghill) said County Farmers the Executors.
The will itself is on-line on the PRONI site.
https://www.nidirect.gov.uk/services/search-will-calendars
This was the only Houston still in Slievenagh in the 1901 census:
http://www.census.nationalarchives.ie/pages/1901/Antrim/Portglanone/Slievenagh/941021/
The 97 year old Ellen Jane Houston is Elizabeth Montgomery’s mother. Elizabeth Houston married James Montgomery in Portglenone 2nd Presbyterian church on 31.10.1870. Her father was Robert, Houston farmer. A connection perhaps?
The fact that John was in Limnaharry in 1851 and Margaret in Slievenagh doesn’t mean that their parents lived there. They might have lived nearby. I notice from Griffiths Valuation that nearby in Drumkeeran, a Robert Houston was farming plot 15a (a 28 acre farm) and in a labourer’s cottage on that same farm was a Joseph Greer. So these might be the fathers of John & Margaret. That’s about 5 or 6 miles south of Limnaharry.
There’s a death for a Robert Houston at Slievenagh on 16.8.1892 aged 84. He was a carpenter (not a farmer). The informant was Samuel Montgomery also of Slievenagh. So Robert was almost certainly married to Ellen Jane Houston. They are of an age to be your Margaret Houston’s parents. But I can’t say that for certain.
You can view that death free on this site:
https://churchrecords.irishgenealogy.ie/churchrecords/
There is also a possible death for Joseph Greer on 4th March 1865 aged 72, registered in Ballymena. That’s not available to view free yet and you need to look it up on the GRONI site (the link I gave earlier). But it might be John’s father.
You ask if local papers in Ireland listed people killed in the US Civil War. The answer is no. Not commonly anyway.