I searched the Irish Family History Foundation's database. It contains rcds for parts of Co. Roscommon. Several parishes' rcds are undigitized.
I found NO rcds for the forename David and surname Henry in any year. Data for this event may be mired in the mass of undigitized rcds.
For marriage rcds, I found an M rcd for Honor McManus in 1854 in Kilronan parish. There is a corresponding M rcd for Patrick Henry in the same yr and parish. It is likely these are your ancestors. A view of either detailed rcd will show if the marriage is for each other. If so, this validates the rcd. The detailed rcd MAY list the names of the parents for both. This would push your search back one more generation.