MikeOz, I hope someone posts here to sort you out in time.
In the interim, let me say that I usually pass Knockbreda Cemetery every week so called in yesterday for a few minutes to talk to the attendant in the cemetery office. All he has locally is the day book containing the owners of the plots and years of burials against them, he advised that any other records that exist would be held by Castlereagh Borough Council.
We looked through the 'B' surnames for Baillies (and variants) and could only find a William Baillie. In the event, when we went to look at the plot, the headstone was for a Dunlop. And of course, there's the problem, the plot owner won't necessarily be interred there - eg. that William Baillie could be a Dunlop inlaw.
There are some 6,500 plots in the cemetery and even taking on board the local knowledge of the attendant regarding where people in the 1910s - 1920s would be buried, you're still talking hundreds of plots. And there's the usual quotient of fallen headstones, headstones obscured by tress and bushes, weathered headstones (I noticed that some headstones as recent as 1940s were quite weathered, no doubt due to the upper part of the graveyard being on a height and facing north over Belfast City).
I can only suggest contacting Castlereagh Borough Council to see what more they can find in the way of records - ideally, if they can find James and Theresa, you need a section and grave number. If you post it here, I'm sure I or others here will endeavour to help you.
Given the time considerations, you could try emailing them, the address is at the very bottom of the page in this link.
http://www.castlereagh.gov.uk/Cemetery.asp