To take advantage of these prices you will need to prove you are a leisure traveller, en route to glasgow-international airport, so you will need to provide either your flight details or your holiday booking reference. I don't think you will need to give this when you arrive at the station but you may be asked to provide them on your journey from the station to the airport.
Going to glasgow-international airport by train is great, you can take a nap, read (even at night), play a game, or just gaze out of the window... The other restriction to buying these inexpensive train tickets from the station to the airport is that there must be at least two adults travelling together. But another great advantage is that children go free. All children under five go free, and then one kidover five but under 16 per grown up travelling by train to the airport.
make sure you leave plenty of time to get to the airport early enough to eat, shop etc, so check the times of trains from the station before you go. If you end up with time to kill at glasgow-international airport you can always read or play travel scrabble again! As you have bought your train ticket in advance, you don't need to fret about the opening times of irvine station.
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