Chris,
The townland name makes all the difference. There isn’t anywhere named Glen Lagan in Co. Down but I think it’s Glenloughan (which would sound much like Glen Lagan to an unfamiliar ear). There’s 3 of them. One in Aghaderg parish (outside Scarva) and 2 in Kilkeel (Glenloughan and Glenloughan Upper).
http://www.thecore.com/seanruad/
Your family seem likely to have come from the Aghaderg townland. It borders the northern edge of Scarva village, and you said you believed there was some connection with Scarva. So I searched the tithe applotment records for Aghaderg parish in 1828 and found 2 Cousins farms, one in Glenloughan and the other in Scarva. Both occupied by a Thomas Cousins. (Can’t say whether there were 2 separate people or the same man had 2 plots of land). See:
http://www.irishgenealogyhub.com/down/tithe-applotment-books/aghaderg-parish.php#.Wi7mvxR2ug0
So that might be your family. (It won’t be your Thomas who was only aged 5 in 1828, but it might be his father). Don’t expect too much when you google these townlands. There’s not too much to them. Glenloughan is about 260 acres of agricultural land. In 1901 there were 19 houses there and a total population of 96.
You noted Thomas Cousins of Aghaderg on the 1796 flax growers list in the Ros Davies site. So that tells you he was an adult and head of his own farm. So probably born before the 1775 or thereabouts.
I looked in Griffiths Valuation for the 1860s but there were no Cousins farms there then. There was one Cousins farm in Scarva in 1901 though:
http://www.census.nationalarchives.ie/pages/1901/Down/Scarva/Scarva/1208430/
Perhaps they may be related to your family. I looked up a few of Moses childrens births on the irishgenealogy site, and can tell you that Thomas Henry was born 1864, Robert in 1866 & Sarah 1869, all in Scarva, and that the mother’s name was recorded as Roseallis Hadden. Some of the children are registered as Cozens, an alternative spelling of the surname.
https://churchrecords.irishgenealogy.ie/churchrecords/
The parents marriage was registered in Armagh 14.7.1853 (Rose Alice Hadden & Moses Cousins). You might want to look at that marriage certificate to see who Moses father was. Moses was born around 1826.
Some probate abstracts from the PRONI wills site:
Probate of the Will of Moses Cousins late of Scarva County Down Farmer who died 24 February 1906 granted at Belfast to John Teggart and Thomas Henry Cousins Farmers.
Cousins Thomas Henry of Scarva county Down farmer died 21 March 1923 Administration Belfast 19 October to Amelia Cousins widow. Effects £139. D.B.N. P.R. 11.11.37
(With the above will it seems the executrix failed to administer the estate and 14 years passed before that happened. Probably as a result of her own death).
Cousins William Robert of Scarva county Down farmer died 25 December 1923 Probate Belfast 20 May to Amelia Cousins widow. Effects £368 3s. 9d.
This Cousins family were Church of Ireland. Perhaps yours were too. There are 2 churches in Aghaderg parish. Aghaderg itself has baptism, marriage and burial records from 1812 onwards. There’s a copy in PRONI. Scarva Church of Ireland unfortunately has no records before 1877, as their earlier records were lost in the 1922 fire in Dublin, during the civil war. But you might find a gravestone in the graveyard. Farmers could often afford a gravestone.
Scarva Presbyterian has records from 1807. It might be worth checking them too just ion the offchance the family were of that denomination. Copy in PRONI.
If you can’t get to PRONI yourself you might need to hire a researcher.
Researchers in the PRONI area: http://sgni.net
I have attached the PRONI church guide to this post, so you should be able to access it that way. (Their site is not very well set out and it is hard to find the guide on it).