Thanks everybody - it's more a generic question than a specific case. And it's started an interesting and enlightening discussion, which is always good.
The specific case that prompted the question was actually resolved a few years ago (details below*), but because I was looking through all the evidence for the family again in a different way I came at the problem from a different direction (and forgot the original explanation!)
The new direction was this: the 1911 census return
http://www.census.nationalarchives.ie/pages/1911/Cork/Mallow_South_Urban/Barrack_Street/431078/ states 13 children born alive, 8 still living. We have 12 civil birth records and 4 civil death records from IrishGenealogy.ie. So wearemissing one child who died before the 1911 census.
But we also have two baptisms three months apart, only oneof which has a corresponding civil birth record. So I was wondering if maybe the first was a premature twin who died with neither birth nor death being registered? But I overlooked two important points - it's the
first baptism that has the matching civil birth, and there's a note on the second baptism that seems to tie it to the first.
For my specific case Occam's razor tells me that the number of children born alive noted on the 1911 census is probably wrong? Or perhaps,less likely, there's a matching civil birth/death for the 13th child that I haven't found because
both were mistranscribed into the irishGenealogy database?
*Because the civil birth and baptism for Johanna Browne are on the same date 28 Aug 1908, and the second baptism (Josephine, no associated civil birth) 3 months later on 1st Dec 1908 has a note "Celeb nr birth date approx" the conclusion is that she had an emergency baptism when she was born, probably because she was likely not to survive, and then when she was in the clear she was re-baptized with a slightly different name - here's the thread where that conclusion was drawn
http://www.rootschat.com/forum/index.php?topic=418092.msg2838122#msg2838122P.S. Baptisms before births is something I came across a lot -
http://www.rootschat.com/forum/index.php?topic=765425