There were several Brennan families living in Copse, Co Roscommon in the mid-1800s according to a researcher I worked with several years ago (verified by the Leitrim-Roscommon site). Copse is a tiny 265 acre townland just West of the City of Boyle. Rather than think of it as a tiny town, think of it as more like a loose grouping of acreages from 1 acre to perhaps 40 scattered over the 265 acres.

Ancestry will not let me attach the Leitrim-Roscommon page http://www.leitrim-roscommon.com/ for the Griffiths Valuation that shows the Brennans in Copse. If you have not taken a look at that site, do so as it is the best I've found for that part of Ireland. The information I have is that the Valuation would have been completed about 1858 but it might help you. There is also a 1901 census and of course, you could look for the town of Boyle as well for the valuation. I've run across a Winifred McDermott in LDS records but she apparently is not related to my McDermottroe's, who have lived in Copse for over 200 years and who still live there today.

You might also check the LDS films #989743, items 1-3 (Boyle, Elphin Diocese) and #1279224, items 7-12 (Kilronan), now the Catholic Church Parochial registers of Keadue, Arigna, and Ballyfarnon (Roscommon), 1823-1984. Both will give you Baptisms, Marriages, and Burials going back far enough to cover the timeframe you need.

Hope all this will help you.