12yr old DD flying solo? Am I crazy?

Absolutely fine IF she's comfortable with it and knows her way around the airport, especially air side.
You know your child best.

Personally I would have let DC1 fly at that age as sensible and engaged.

DC2 on the other hand...

Does she have a SIM card that works in both countries?

I wouldn't have a problem with this.
I don't know which Irish airport you use but none are huge and are fairly easy to navigate. If her dad is on hand for delays/cancellation that's not an issue.
As to personal safety while on board: it's a short flight, not a night flight where a lot of people are asleep, the staff will be up and down with sales for most of the flight. A bit of prep on approaching staff if you're uncomfortable and I'd be good to go.

Make sure she has a consent to travel letter with her if needed.

I flew at 12 with my 8 year old cousin. I’d only flown once before, but it was all fine. Many years ago and flying was an adventure. I’m sure she will be fine and enjoy it. It’s not a long flight.

During college i worked in a travel agency in London. We had many kids fly solo, airlines are notified and the crew there to offer extra help where needed.

I think it’s just a general conversation to have, not connected to this trip specifically - it applies just as much if she’s taking the train or bus back in Ireland.

She'll be fine. Aer lingus is a great company, if there is any issues they're helpful.
Will she be okay going through security herself? It can be daunting.

I'd say "If a person next to or near you on the plane says or does anything that makes you uncomfortable, tell a steward immediately."

I flew alone and without airline assistance regularly from 12 (also alone from age 10 but with the airline looking after me.) For a short, direct flight as you are sugesting I would definitely allow it. Having got caught up in the Qatar Airways shambles in June I would not risk it if there is a connection.

The bottom line is that nobody can guarantee anything for any flight. It will probably be fine, but there are potential issues - a bit like most aspects of life really.

This.

How exciting, she'll have a great time. I bet her family at the other end will be so happy to see her.

My dS was 15 and Aer Lingus were great. They don't do accompanying but they do look out for them and tend to be very nice and helpful. We were a bit worried until the London cousins suggested meeting him outside a landmark near a station, so basically he had to find his way around Heathrow, navigate the tube system and then find the right exit and walk to meet them. It made the flight seem ridiculously easy by comparison! He was fine, we delivered him to the airport and he understands the security process. At 12 if you've someone meeting at the other side you'll be able to bring them as far as security on your side so it's not that much inbetween really, just getting to the right gate. If she has a phone she can send you a pic of the gate screen and gate when she arrives, if it makes you both feel better.

I think its very exciting and a wonderful way to becoming an independent adult.

Ahh this sounds like an amazing adventure for her, and it sounds like she's confident she can do it. I can completely understand your concerns, and all the 'what ifs' but chances are none of them will happen, and you can coach her for most situations. I think air travel is very safe and airport staff and cabin crew will be very supportive of her if she needs any help.

I agree with PP about booking her an aisle seat. I'd also probably book her near the front of the plane if possible so she's close to cabin crew.

@Flyingfear85 sounds like she is pretty confident. I would just reiterate that if anyone does anything she is anxious about, or she feels unhappy generally, that she press the cabin crew ‘call’ button and politely ask to be moved and it will be ok. I am sure she will be fine though, I have friends who split up whose kids used to fly regularly between the parents (admittedly two kids) but that was long-haul!

She sounds like a lovely girl and level headed. She's going from a small airport to another small airport and being met at the other end by people who love her.

So there is no chance they will let her down and will actually be there.

Let her do it. It's not a long haul flight to a foreign country. Even if the flight is diverted she has a phone to call you so you shouldn't worry.

Well you will worry, I know but let her do it.

Tell her that if someone next to her is bothering her or being inappropriate in any way, she should get up and go to the end of the plane as if she is going to the toilet, and tell one of the staff.

If she’s done it before and feels confident I wouldn't worry. (When I first saw the headline of your post I thought you meant she would be flying the plane herself, which would be a different story!)

@Flyingfear85 I'm afraid that I don't believe you're doing your DD any favours by trying to hide the realities from her, especially if you are ALSO letting her have independence. The sad fact is that as a girl/young women, she will experience some version of sexual assualt/harassment, and she needs to know how to deal with it and have the confidence to say NO.

With both of my DC, I've made it clear that if adults or other people them uncomfortable, say things that seem too personal/inappropriate, touch them, or ask them to do things/go placse with them, it's 100% okay to say NO and/or to run away, or ask someone else for help. I can talk to them about this without discussing detailed sexual matters. DD, for example, udnerstands that sometimes men can be a little odd and like to get a bit too close to girls/teenagers and that really, that's not okay, and if it happens, she should shout or run away. We emphasise that most people are normal, kind, caring people, but that there are enough of the ones that are a bit off that she needs to know.

Both DC did martial arts when younger and the teacher was very firm on teaching them that they had every right to bodily autonomy and to feel comfortable. His self defense lessons prioritised running if they were uncomfortable, or going to another adult.

I've also always encouraged my chidlren to understand that "official" people are there to help - from police to the person working in Sainsburys.

So for your DD, the key thing is that she knows that if anything ghappens she's uncomfortable with, she should speak to the air steward. I'd also recommend aiming for a aisle seat for her.