What a sad and disturbing thing to discover after your wedding. I’m so, so sorry. You must be angry and confused and terribly disappointed. From your description of the situation, I do think you are right to be concerned.
You are absolutely correct that it is very wrong for your husband to be attracted to underage girls and to be seeking them out. He knows it too. First, he hid his interest from you and now he justifies it. I don’t know anything about Irish law, but even if it isn’t illegal to look at pictures of the young teens, I imagine that their parents would be upset and furious if they knew. The kids may not have good judgment about what they are putting on FB but he is a 26 year old man. Mature men help keep a community safe. They don’t contribute to the sexualization of children and teens by adults.
The most hopeful sign is his willingness to go to therapy. I also think it will be important for you to be included in some of his sessions. You will need reassurance that he is able to make a fundamental change in his sexual interest for you to trust him and for the two of you to ever have a satisfying intimate relationship and a successful marriage.
I wish you well.