I search the IFHF's online database at ROOTSIRELAND.IE. It has abt 20 M church/civil/census/etc B/M/D records for MOST Irish counties. Records before 1863 become sparse - almost non-existant before 1800. Protestant records were the least likely to survive.
For a Mi(chael) Gallagher and Mary Finan marriage 1826 +/- 5 yrs, Co. Roscommon, I found 0 records.
For Hugh Gallagher B 1830 +/- 1 yr, Co. Roscommon, with a father named Mi(chael), I found 1 record.
For Cat(herine) Gallagher B 1832 (exact), ditto, I found 1 record.
For Th(omas) Gallagher B 1838, ditto, I found ditto.
These appear highly likely to be for your ancestors. They fit your parameters nearly perfectly. Views of the detailed records (at small fees), should validate them and show other info.