Is it legal to take intoxicants at an airport or on an airplane?

If it's a prescribed drug and it's legal at both ends and all transit points of your trip, it's fine to take on board, just check Customs regulations beforehand and bring your prescription with you. I would not recommend trying out a new medication on a plane for the first time though, and combining both Xanax and Ambien sounds like a bad idea.

FWIW, I was also prescribed Ambien for a long flight once. No particularly dramatic side effects, but instead of sleeping properly I felt like I was drifting in and out of consciousness, without being able to control when I slept, and felt like a complete zombie while I was awake. It was rather disconcerting, I didn't feel rested on arrival, and I would have been quite useless if there had been any emergency/diversion/etc in flight, so I never tried it again. YMMV.

lambshaanxy's advice is all spot on. A few additional tips:

• Try it out sleeping on the ground and see how it works for you first. If you're uncertain about anything, speak to a doctor and/or pharmacist or go without on the flight.

• Not only does combining Xanax and Ambien sounds like a bad idea, it sounds like something you shouldn't do absent clear instructions from a doctor or pharmacist that you're taking two drugs that interact and a full understanding of how it works for you on the ground. Similarly, don't mix them with alcohol.

• Take the medication and go to sleep. Don't plan to try to use the lavatory or anything while you're waiting for it to start working.

• Know how long Ambien lasts. It can come in extended release formulations that last longer (to help you stay asleep). Depending on your size, metabolism, whether you've eaten recently, etc... they could last for quite a while. You naturally want the effects to have largely worn off before you land, so stick to taking it early in a quite long flight to be safe until you know how long the effects last for you, especially if you have the extended release version. Since this trip will involve 2-3 flights, you'll need to schedule when you plan to sleep to ensure you can take the medication when you still have many hours in the air left to go.

• Some countries, especially in the Middle East, have strict rules about narcotics, even prescription medication, and you should be sure it's permitted for you to carry these even if you just have a layover.

I've read your question and I know for sure these drugs (Xanax and Zolpidem) are restricted when entering the UAE. There are specific rules about bringing medicines and medical devices into another country. Many medicines and medical devices can be brought with you into Australia as long as they are for you and your personal use. Some require a permit and others cannot be brought with you at all.

Prohibited items are Aminophenazone, amidopyrine, aminopyrine, dipyrone, metamizole, Amygdalin/laetrile

It is better paying a call to the Australian embassy in Ireland than having trouble at the airport.

Also, please read carefully on the Official Australian Government webpage about travelling. https://www.tga.gov.au/entering-australia https://www.odc.gov.au/travellers