It's not easy when you got ancestors with common surnames!I had a similar problem with with my Kavanagh ancestors, and ended up with quite a number of incorrect certs before I found the ones I was looking for. Although I suspect O'Sullivan could be even more difficult..The one detail that the Index books show that isn't in the online Index on Familysearch is mother's maiden name in the earlier years. The online index includes this detail from about 1927 - I believe the Index books show this from an earlier date.. poss. 1905 ?Shane