You haven'rt really given us any idea of dates here but there's a civil registration which might be Harriet-
Harriet Eveline McMahon July/Sept.1915 Cookstown district, volume 1 page 455.
registration district: Cookstown
Possibly Monica in Births-
Elizabeth Monica McMahon July/Sept.1914 Cookstown district, volume 1 page 517
The online civil registration index only goes up to 1921 for Northern Ireland counties.
Many people didn't have headstones or sometimes their names are omitted from them- check with the church or cemetery to see if they have any burial records.