Annarose, I believe it's so they know what you have and don't have floating around in your system if you end up getting more pain relief in the hospital or needing a CS. Also, I suspect because they fear you might lose count of how many you take.
As well as that, they need to gauge how your contractions are feeling without pain relief so they can tell when you might need to start thinking about heading to the hospital. One of the ways a doctor or MW can judge how you are doing is by asking about your level of pain and hearing how you speak over the phone - whether your voice is affected by contractions, etc.
And there is also the fear that well meaning friends or relatives would dig into their pain relief supplies - including prescription pain relief - and prevail upon a woman to take something effective that might not be good for the baby. If there is an interdiction on even acetaminophen then it's easier for a woman to refuse to take the codeine her well meaning friend has in her medicine cabinet.
Minifingers --On the other hand, if you're willing to spend some cash, as you would in the US, you can have a gold standard service - one to one care from a private midwife all the way through pregnancy, birth and the postnatal period with the back of of NHS obstetric and neonatal services should things become complicated or dangerous.
Or you can get gold standard service while on Medicaid and not have to pay one cent for it -- one to one care from your own personal MW or doctor all the way through pregnancy and delivery and afterwards as you recover in the hospital, and all the neo natal care you could hope or wish for including an examination by the hospital pediatrician as a matter of routine, or neo natal specialists if the need arises....
And as I mentioned before, clean, clean, clean hospitals -- clean room and clean ensuite shower and loo, clean furniture in your private or semi private room, clean sheets daily, clean floors and doors and windows and TV remote, all the supplies you could hope or wish for all laid on, for yourself and your baby...
And I know we are not talking about Ireland here, but I know someone who had to have an EMCS and was on the trolley being taken up to the next floor (terrible hospital design) where the theatre was; she and her MW and her DH waited and waited and waited for the lift, everyone getting more and more frantic as ten minutes passed and no sign of it. When it finally arrived and opened a bloody meal service crew emerged from it. There is no way on this earth they should have been using that lift. It's a well known fact that your chance of receiving even passable care in Dublin maternity hospitals is slim to none if your due date falls between April and October, the busy baby season.