Thanks Sinann for replying. This is the entry for Mary Alice ... as someone pointed out, the rest of the pages list parents by 'father & mother surname' whereas her entry has parents as "William PLUNKETT and Catherine SMYTH". This is why I'm possibly wondering if they weren't married....
Having now looked at other entries on those 2 pages, I think the differences in format depended on which priest baptised the baby or entered the baptism in the register. Some entries include 'P.P' which is presumably Parish Priest. Examine all those with 'P.P' and you may find that they follow one format whereas those without 'P.P' may be different. Names of priests weren't included in this register so we don't know how many priests were in the parish.
Some baptism registers with headed columns were completed to a set format and it's apparent what each piece of information is. Others were more random and each priest seemed to decide for himself what information he recorded and how he wrote it. When the priest changed, method of register keeping might change.
I don't think you can judge from this baptism register if Catherine was married to the father.
I noticed surnames Smyth and Plunkett at other baptisms.