Eddie,
Save for a few fragments, all censuses pre 1901 in Ireland have been destroyed.
I found Margaret Brown’s death in 1881 at Ednego, aged 26. You’ll note that she had a daughter that died a few days previously. Both deaths on the same page.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1881/06432/4848122.pdf
So siblings for Samuel b 1874 were:
William 14.7.1872
Edward 20.6.1876
Isabella 22.7.1879
Un-named female 26.8.1881
James remarried in 1892 to Annie Spratt:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1892/10647/5879301.pdf
Here they are in the 1901 census:
http://www.census.nationalarchives.ie/pages/1901/Down/Dromore_Rural/Drumbroneth/1205768/
Family in 1911:
http://www.census.nationalarchives.ie/pages/1911/Down/Dromore_Rural/Drumbroneth/207624/
The Valuation Revision records on the PRONI site show James Brown in Drumbroneth. He had plot 5h which was a house and small garden. His name was deleted and the house vacant for a while. I think he left in 1911. There was an Alexander McKee there by 1917.
Family were Church of Ireland (Episcopalian), so possibly buried in Dromore Cathedral graveyard. (But they might have had a family plot somewhere else).
There are several trees on Ancestry which have this family. Some have some errors on them so need to be taken cautiously but one (Ericksen) has Isabella dying in Glassford, Lanarkshire 25.5.1972. It also has James Brown as having sisters Isabella Maria Brown b 11.1.1854 Dromore & Matilda b 21.3.1858 Dromore. No indications as to what became of them. The tree also shows Margaret Jane Graham dying in Ireland in 1915 but that is wrong. She died in 1881. (Confirmed by your Samuel’s 1900 marriage certificate which shows her as deceased).
If you have an Ancestry sub you might want to get in touch with with the tree owners and share information.