Ah I know how you feel, in work I changed to my married name straight away but doc had me as my maiden name!!! So what I did was called myself babsdel (maiden name then married name) I used this on all correspondence with maternity and with hospital etc etc. My midwives then asked me themselves am I going to use both names when babs comes, so I told them no just married name and they changed it on the clinic card themselves. I don't think it really matters too much to be honest, just pop in a copy of your marriage cert with your form. That's what I did anyway.....hope this helps a bit
The MB form asks for your married and maiden name if applicable afaik. I'd put down whatever the Revenue has for you to avoid any confusion.