Can I stop my partners girlfriend of 1 year seeing my child on the day's he was granted for access. I have no specific reason to tell the court I'm just jealous of their relationship

No, you cannot.

No, if you try, you may even lose custody of the child to them if you keep doing it, and they take you back to court.
At the least you will be held in contempt of court till you allow visits.

I have had the court date and the father of my child has agreed to keep the child away from his girlfriend if he got his access. The follow up date in sometime in April. I think he thought he had no other choice, can he take me back to court? I'm in ireland so does anyone know the law here?