Ugh.
You may never be certain she went to Italy. You would have to trust her. Nothing says her former BF couldn't have met her in Italy. You won't know if she played around even if he wasn't there.
This relationship has to change or else it's doomed. Insecurity and possessiveness kills relationships faster than anything else short of murder.
First, work on your self-esteem. You're a good-looking, you've got a big dick, and seem like a nice guy. That's a hell of a lot more than most guys have. It's normal to be a little jealous or even slightly suspicious, but rather than dwell on it and become self-destructive, you should be able to gather your confidence and trust and let go of your worries, enjoying the time you have on your own.
That you can't indicates either you have a real reason to be suspicious, a reason you're not telling us, or there's a fundamental flaw in your relationship that you both need to address. It's not unusual for women to have a bf but flirt a lot more than they would if they didn't have a bf. Some women can't resist using jealousy as a means of controling their boyfriends. They do this to demonstrate their power in the relationship. Conversely guys may do the same thing, but it's less usual as guys tend to be more possessive than women. It's circuitry. Instead, guys just get girls on the side.
Your jealous reactions aren't going unnoticed. Women are masters of emotion and will be able to sense when what they do or say will make you uncomfortable. You say this trip is a vacation, rather than a family/friend visit. If that's the case then there was no other need for her to go to Italy other than to relax and have fun. Did she invite you to go with her or express how much she'd like you to come with her?
If she did, great.
If she didn't, then she's likely using this to demonstrate her independence. She wants to re-evaluate things in her life and wants to know that you trust her while you're not around. When she returns, tell her that you missed her and hoped she had a good time then tell her about what fun you had while she was away but, in the end, you're happiest she's back. That's the emotionally healthy response. She won't want an interrogation, demands to see her passport, her pictures (you've got to bring a camera to Italy), or anything else. If you do that, you've blown it. It means you don't trust her and that's that. If she makes no effort to contact you (no postcards/emails from icafes), then I would strongly suspect this is the situation.
She's also using this as proof of her trust in you. She's demonstrating that she trusts YOU while she's away. When she comes back she'll be looking for signs that you weren't playing around either. The idea will at least cross her mind.
Her reaction to you upon her return will tell her about you as much it will tell you about her. Either she really met her former bf and will decide she really loves him and dumps you.
You can't control that outcome so don't stress over it too much. Better to find out now rather than later, painful as that may be.
or
She returns, you're cool with her, say and demonstrate your respect for her independence, your joy on her return, and maybe drop a slight hint that you thought this time apart was good for you both. Let her know how much fun you had. Make her wonder just a hint that maybe you had too good of a time so she doesn't take you for granted because once that happens, then she knows she controls you and your status descends into lapdog territory. As much as she makes you work for the relationship to succeed, so should you make her work. That's what keeps both of you coming back to each other.
Right now, your anxiety demonstrates there's an inadequate amount of trust in the relationship. You mentioned that she's had contact with Mr. Ireland while going out with you. She may still love him or just be using him as a means to make you jealous. That has to stop. It won't stop, however, by you demanding she not contact him again. Work at building trust, not possessiveness, between you two. If you do the same thing with other exes, then stop. She will trust you more when you become more confident in yourself and your relationship. You will trust her more when she does the same thing. I think this trip is an attempt on her part to do that. You have to match it. Women are usually a step ahead in this kind of thing. They see problems before men do. Circuitry again.
If after all this it turns out things didn't work and you go separate ways, then use that time to get your life together. Work on future plans, do productive things with your friends, focus on school, spend time with family, or anything else that helps you move-on and become secure in your life. It's tough being a student. You're waiting for real life to come along and probably suspect real life ain't the one on Mtv. Best thing is to work out what you can, look at what you're doing with your life and work to your goals. Yeah you'll be depressed and upset but fight that. You'll think a lot of things about herself and yourself in the fit of the moment but don't let those emotions get ahold of you. When my mother divorced my dad he never recovered. He let those feelings eat away at him and he's been depressed and feeling bad about himself ever since.
That was 25 years ago.
The most sexy, attractive thing about a person is their confidence. You're aided by having a good education, good looks, and a nice personality. My dad had those things until he let depression destroy his marriage and his life. He used that negative attention as a means to get all attention and it just drained everyone around him to the point they couldn't stand being with him, my mother included. We advised him to get counseling, maybe try pills, but he refused. He liked being the center of negative attention. I visit, as does my sister, but we can't stay long. The black cloud is toxic.
Don't become that man. Build your confidence. Work at it. Say to yourself, "What can I do to make myself feel better?" It's a fight. Sadly, life is such that you have no choice, but to fight it yourself. You will get encouragement and support from your friends back at the barracks, but in the end it's you alone up in that plane dogfighting your own demons.
Good luck and keep us posted!