Stephanie:
Welcome to ireland Reaching Out!
The subscription site Roots Ireland actually has baptismal records for five children born to John Gilmartin and Catherine Dempsey. Besides James there were records for Michael John Ann and Brigid. See transcriptions below. I did not locate a marriage record. Possibly Catherine's record was missed in transcription. I will look at the register.
By the way, I looked at the 1855 Griffiths Valuation head of household listing for Kilteevan civil parish and there were four Kilmartin records including a John in Aghmagree townland. Possibly this is your ancestor. https://www.failteromhat.com/griffiths/roscommon/kilteevan.htm
Roger McDonnell
Name:Michael KilmartinDate of Birth:
Date of Baptism:07-Feb-1836Address:Not RecordedParish/District:ROSCOMMONGender:MaleCountyCo. Roscommon
Denomination:Roman Catholic
Father:John KilmartinMother:Catherine DempseyOccupation:
Sponsor 1 /
Informant 1:Peter KilmartainSponsor 2 /
Informant 2:Mary Kilmartin
Name:John KilmartinDate of Birth:
Date of Baptism:28-Jun-1841Address:Not RecordedParish/District:ROSCOMMONGender:MaleCountyCo. Roscommon
Denomination:Roman Catholic
Father:John KilmartinMother:Catherine DempseyOccupation:
Sponsor 1 /
Informant 1:Terance KilmartinSponsor 2 /
Informant 2:Catherine Noon
Name:Ann KilmartinDate of Birth:
Date of Baptism:25-Feb-1844Address:Not RecordedParish/District:ROSCOMMONGender:FemaleCountyCo. Roscommon
Denomination:Roman Catholic
Father:John KilmartinMother:Catherine DempsyOccupation:
Sponsor 1 /
Informant 1:Michael KilmartinSponsor 2 /
Informant 2:Catherine Hunt
Name:Brigid KilmartinDate of Birth:
Date of Baptism:14-Nov-1848Address:KilteevanParish/District:ROSCOMMONGender:FemaleCountyCo. Roscommon
Denomination:Roman Catholic
Father:John KilmartinMother:Catherine DempsyOccupation:
Sponsor 1 /
Informant 1:Patrick RocheSponsor 2 /
Informant 2:Ann Kilmartin