The 1901 & 1911 Irish censuses show individual religions clearly. From that it is not too difficult to find mixed marriage families and equally not too difficult to find some where, as you say, some children were brought up under one religion and some under another. (The sons often following the father and the daughters the mother).
Church authorities can issue all the edicts they like but sometimes human nature quietly goes it's own way and ignores rules that don't suit. Couples may have given certain undertakings at the altar regarding the future religion of their children but if, when it later came to the bit, they decided differently and actually only baptised half their children into that religion, what would actually have happened? Would the church have excommunicated the whole lot? I doubt it. It seems to me that regardless of it's stated position on the matter, in reality, the RC church generally accepted a fait accompli that was somewhat different.
Elwyn