The full force of the law would be against you and the child would be brought back to the sole-guardian.
Terri, alas, it doesn't work like this in Poland. I know of at least four different cases off the top of my head where the courts have played endless stupid games when dealing with a situation where a child should be immediately returned according to the Hague convention, including one situation where they simply kept demanding more and more reports until the father gave in. Another situation had the police claim that the child couldn't be found, despite the father presenting the courts with a detailed dossier of evidence showing exactly where the child was, where the child attended school, where the child attended private lessons, etc etc.
There's a good write-up here of a typical case: internationalfamilylawfirm.com/2018/01/polands-further-violation-of-obligation.html
As much as it pains me to say this, the odds of anything happening to the mother if she does kidnap the child are zero. Even if the child is taken by the police and returned to Ireland, the mother won't face any legal consequences here.