Penton:
I'm not going to advise you one way or another. However, I can pose some scenarios that may help you decide:
Firstly, along the lines of what ButtLight said, you can ask him if in the future, he would leave you for the Organization if he had to make a choice. That should tell you something. Of course, that depends on his motives for wanting to marry you in the first place. I'm not talking about whether he loves you or not. I'm considering, as you should, what's on his mind. As you've learned here, JWs are strongly discouraged from marrying outside the faith. So, maybe he hopes that he'll bring you in eventually and shed the stigma later on. That's when your marriage can go like a ripe avocado hitting the ground, SPLAT!
On the other hand, his faith may be wavering and he's looking for an excuse to leave or perhaps he doesn't care what happens to him because he cares more about you. Either way, some people experience a lot of guilt when they leave and end up returning. In that case, the first scenario I painted may occur and he may leave you because he needs to get back into the religion and you don't. Here is a good example: http://www.jehovahs-witness.com/7/46758/1.ashx and http://www.jehovahs-witness.com/6/53595/1.ashx
What you need is what every couple contemplating marriage needs: TALK EVERYTHING OUT FIRST BEFORE GETTING MARRIED. You don't want to wait until you have children. I learned about an interfaith marriage (Christian and Jewish) that made a very good couple and were very happy until they had a child. The issue came up as to what faith they were going to raise the child and the disagreement ended their marriage. I guess each didn't think about it because they assumed that the kid's faith would be their own.
And oh, forgive me if I'm being too forward, but you stated that you're Irish Catholic. Well, I presume that the picture of a good Catholic girl applies to you and you haven't "done the deed" with him. If that were the case, you must realize that he can be expelled from the Organization, it would be announced at the meeting and he would be shunned. That can be devastating for a person unless he or she is ready to go.
Discuss everything. Assume nothing. Know his stake in the relationship. Learn about the JWs so that you can talk to him intelligently.
Etude.