Many thanks for your query to the message board and welcome to the site. I have had a look for you as to Patrick Foster on the Roots Ireland website, but unfortunately most of the records for the surname "Foster" appear to read as Forster and are Church of Ireland records. Do you have a father's name for Patrick? That might narrow your research and also give an idea of where he may have come from in County Monaghan. I also tried searching for James Foster and John Foster with the same results.
With respect to Sarah Brady, I did not find a baptism record for a Sarah Brady with parents of Patrick Brady and Julia Brady in County Fermanagh on the Roots Ireland database. It is quite possible in both cases that the registers are not on the Roots Ireland website but are available on the National Library of Ireland website. In that case, it means that you would have to search through those registers that are not on Roots Ireland. I also checked the Tithe Applotment Books online through the National Archives. These books consisted of listings of those occupiers of holdings who paid tithes to the Church of Ireland. These books cover the years 1823-1837. There is a listing for a Patt Brady in the parish of Tomregan,and this may be a good place to begin to search. Unfortunately, the parish registers only begin in the 1860s, but they are online at the National Library site. Here is the link: https://registers.nli.ie/parishes/0819 Here is the link for the Tithe Applotment Books: http://titheapplotmentbooks.nationalarchives.ie/search/tab/index.jsp You can also use the links for County Monaghan to search for your Foster ancestors. Incidentally, when I clicked on the Tithe Applotment Books for County Fermanagh and Foster (my mistake) the only Foster that came up was also fromthe parish of Tomregan. If you have any further information,no matter how inconsequential, please post it here on the thread. There may also be others out there who are searching for the same families, and you might consider creating an Ancestor Profile for one of the great-grandparents. The XO Chronicles link is: https://www.irelandxo.com/ireland-xo/history-and-genealogy
The very best of luck with your research.