I looked around a bit more and James' wife was also named Catherine. This would mean your examination of the tombstones is critical. Were the tombstones for the Catherine and Bartholomew mentioned earlier indeed the same (i.e. indicating they were the corresponding Mother and Father).
Stella is the daughter of James O'CONNOR and Catherine BREEN. This calls in to question the assumption that Bartholomew purchased the plot for Catherine, Stella and James. It is more likely that James purchased it since there is another Catherine O'CONNOR buried in nearby.
This is why I cautioned earlier about the assumptions of the database. The two families are still buried close to each other so pursuing the sibling link between Bartholomew and James is still valid.