I searched the Irish Family History Foundation's database. It contains parts of surviving parish rcds for Co. Roscommon. Four entire parishes remain undigitized. ALL rcds B4 1840 are sparse.
For Patrick McGowan (+variants) B 1826, I found 1 B rcd in Roscommon parish. It is for the "Smyth" variant of this name. This rcd MAY, or MAY NOT, be for your ancestor. The chances are less than 20% it is. A view of the detailed rcd will list the exact birthday, and MAY reveal parents names. Either can validate/invalidate the rcd.
For Mary Havey (+variants) B abt 1827, Roscommon, there are no B rcds prior to 1839 except for 1 with an indeterminant date. Hence, there is no clear match. For an all Ireland search, there are 2 B rcds for 1827, both in Co. Armagh. Armagh is a long walk from Roscommon.
For a marriage between Patrick and Mary 1850 +/- 4 yrs, I couldn't find any rcds for each that matched on the year, county, and parish in either Roscommon or all Ireland. The rcd may still be in the undigitized mass.
Send a query to email@example.com regarding the undigitized parish rcds. "All baptism records held at the centre, can be searched by the staff there, please contact the Centre directly for a quotation."
Google The Foundation's name to lead you to the database. Good luck. Make a post to let me know your search results.