I searched the Irish Family History Foundation's database. It contains 11,500,000 surviving rcds for abt 60% of Irish counties. Some Co. Roscommon rcds are in it. Several entire parish rcds remain to be added. Rcds for Kilglass parish begin in 1865, probably due to mandatory civil registration laws. All rcds B4 1840 are sparse.
For John Igo (+variants) B 1814 +/- 4 yrs, I found 0 B rcds. For Mary Duffy (+variants) B 1814 +/- 4 yrs, I found 5 B rcds. However, none were in Kilglass parish.
For a marriage between the two, I found no instances of corresponding M rcds for each in the same parish in the same year.
I conclude the events were never rcd'd, rcds have been lost, or rcds haven't been digitized yet.
You may want to contact firstname.lastname@example.org to ask when rcds for the missing parishes will be added. Meanwhile, google the Foundation's name to get to the database and perform your own search. Registration and browsing are free.