I would think the first step would be to check for Seville baptisms in the same parish as the marriage in later years. I dont imagine that Seville is a common surname in the area, which should make the search easier.
You didn't mention a religion for the family, but one detail worth noting is that the parishes gradually altered over the years - e.g. areas previously included in the large Booterstown RC parish were moved to parishes of Dundrum and Blackrock...
Do you know any more about James and Sarah - e.g. where in the parish the lived at the time of their marriage ?
Shane
