On Plan B, you have two elements of cover for obstetricians: in-patient (for the delivery of the baby) and out-patient (for the visits, etc).
Usually, the obstetrician factors in VHI's in-patient fee and charges you the rest. He/she then bills VHI directly for the in-patient element after the baby is born and this paid directly to the consultant. You might have seen this on the claims statement that VHI would have sent after the claim was paid.
Plan B does include limited out-patient cover and €255 of the obstetrician's bill can be included with your annual out-patient claim (as well as other consultants, GPs, etc). However, there's also an excess (currently €300 per member) on this cover, so you may not get anything back once the excess is deducted.
Also, according to their rules, out-patient claims must be submitted within three months of your renewal, so depending on when your last child was born and when your renewal was, you may not be able to claim anyway. I don't know how stringent VHI are with this rule, so it's no harm ringing them to see if you have any leeway.
On the plus side, if you're a PAYE worker, you can get tax relief on medical costs not covered by your insurance, and you can make a claim for up to four years back. For expenses incurred in 2009, the refund will be 20%, and for expenses incurred in previous years, the refund will be at your highest personal rate, depending on how much tax you paid in those years.
More info here.