It is always open to private individuals to enforce the law by bringing private prosecutions. You also have the option of taking the HSE to judicial review if they decide not to prosecute a particular case. However, I agree that neither solution is satisfactory if HSE is failing to enforce an important law.
Going back to the question of criminal cases, I doubt anyone will face prosecution. One of the basic principles that is generally followed in criminal law is that, with a few exceptions, you can only be convicted of an offence if you were aware, or should have been aware, that what you were doing was wrong. Most of those involved can probably argue that, prior to the SC ruling, they had a reasonable belief that they were required by law to admit trans women to single sex spaces for women. If the courts accept they genuinely believed this and that the belief was reasonable, it is unlikely they can be convicted.