Mark Madill died at Tattincake on 6.7.1879 aged 74.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1879/06505/4873245.pdf
Jane looks to have died in 1867 aged 72. Her death certificate is not viewable free yet. If you want a copy you would need to order it from GRO Roscommon.
You haven’t said what denomination your families were, but judging by the 1901 census most Madills in Monaghan were either Church of Ireland or Presbyterian.
Tradition was to marry in the bride’s church. If Jane did marry twice it could be in different churches as she may have switched after her first marriage.
Tattincake is in Currin parish. The Church of Ireland (COI) in Currin itself has baptisms from 1835 and marriages from 1812, burials 1816 onwards. The COI in Drum has baptisms from 1828, marriages 1829 and burials 1842 onwards. There are 2 Presbyterian churches in Drum but neither has any records before 1867. The local RC baptisms start in 1845 and marriages in 1864.
So if Jane was married previously, unless it was in Currin Church of Ireland, there probably aren’t any records of it.