I did, my then partner had been separated from his wife for 10 years and they hadn't divorced as it was amicable and neither initially had the money and then he wanted to make sure she had the benefit of his death in service benefits should he died as they had kids.
We got engaged and an amicable divorce proceeded at our expense. And the ex-wife came to our wedding! Getting engaged if you have been separated for ages is understandable but getting married is not. This is him acting inappropriately and not you.
I believe that in the Republic of Ireland, it is not possible to divorce your partner without their knowledge. Divorce proceedings require that both parties are informed and have the opportunity to participate in the process. The spouse initiating the divorce must serve legal documents to the other party, ensuring they are aware of the case.
If you suspect that your partner has filed for divorce without your knowledge, you can check court records. You would need to contact the relevant court office where the case might have been filed to inquire about any proceedings involving you. I would check if you are divorced without knowing it, he could have lied.
And If you have his address, I would urgently send him and his partner a congratulations on your engagement card (by recorded delivery) saying you wish them all the best and that you assume he now wants to initiate a divorce so he can remarry and that you would be happy to expedite this.
I would address it to them both, not just him as she clearly has no idea. If you are divorced, he needs to explain that. If you are not divorced and he goes ahead with the wedding, you need to tell the Garda exactly what you believe. The poor woman he is with, likely has no idea what she is getting herself into.