Answer: In the winter time, most flowers would be supplied by florists and they would be the best qualified to help you determine which would last the longest -- and what they might have available at that time of year. They might also be able to recommend a florist in Ireland who could supply what you need locally.
Even when you have selected typically long lived blooms, their freshness would also depend on how long they have been in the retail process, meaning how long it has already been since they were first picked, and how they have been handled since then in terms of conditioning and temperature. Usually, one of the most important factors for longevity of cutflowers is temperature, with cooler (below 60 but not freezing) being better.
I would also suggest you consult with the authorities in Ireland that regulate and control importation of plant material because there may possibly be restrictions on them. It would be terrible to be forced to leave the flowers at the airport due to import restrictions.
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