Chris,
I think you may have 2 different Abraham Ervins mixed up. There was one from Co Antrim (father Robert) who married Ann Douds (Dowds) in 1865, and another from Co Down who married Elizabeth Ann Brown in 1860.
This looks to be Abraham Ervin’s marriage to Elizabeth Brown in 1860. They married in Annahilt Presbyterian, in Co Down. (Tradition was to marry in the bride’s church so that may well be where the Brown family attended church). Abraham’s father was James Erwin (not Robert) and Abraham lived at Drumlough. Elizabeth at Magheradartin and her father John Brown was again a farmer.
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_re…
Griffiths Valuation c 1860 lists Abraham Irwin in Drumlough. That suggests his father was dead and he had taken over the farm. It was plot 13a, a 28 acre farm. (On the modern Drumlough Rd. Still a farm today looking at it on Google Earth).
James Irwin is listed in the 1834 tithes as farming in Drumlough:
http://www.irishgenealogyhub.com/down/tithe-applotment-books/dromore-parish.php
Griffiths lists a John Browne in Magheradartin. He had plot 24 which was a house & 44 acres, and also another 11 acres on plot 28.
Annahilt Presbyterian church has records from 1801 onwards. There’s a copy in PRONI in Belfast. They may contain Elizabeth Anne’s baptism and that of any siblings.
John Brown died in 1895 aged 70. His wife was still alive. Daughter Jemima was the informant:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1895/05918/4680277.pdf
This may be Jemima’s marriage to Robert Gilliland in 1902:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1902/10281/5741740.pdf
A Christina Brown, farmer’s widow died in Magheradartin in 1914. Not clear who her husband was though he was dead by 1901, so she's a possible wife for John Brown:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1914/05298/4473221.pdf
http://www.census.nationalarchives.ie/pages/1901/Down/Ballyworphy/Magheradartin/1240612/
PRONI’s wills site has 10 wills for Brown of Magheradartin. There were several farms on the townland so some research may be required to work out who was related to who.
Probably your first step to trace the Brown family would be to search Annahilt Presbyterian’s baptism records for around 1841 when she was born, to see who Elizabeth's mother was. (Elizabeth gave her age as 19 when she married in March 1860, so should be baptised around 1841. Search 5 years either side as not all ages were accurate and not all baptisms were undertaken immediately after birth).
If you are unable to go yourself, you could employ a researcher. Researchers in the PRONI area: http://sgni.net
Here’s the marriage for the Co Antrim Abraham Ervin (Erwin/Irwin) who married Ann Douds:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_re…