The problem is that in the UK, while the cards are now branded as Maestro, they're still processed via the old style national debit card system which used to be known as Switch when they're used for UK transactions.
So, basically, they're not really accepting international maestro cards at all.
When a UK Maestro card is used outside the uk, they're normal maestro.
So, if you try to use an Irish, or any other non-UK Maestro card on a UK site, it will decline the transaction.
As yet, Maestro isn't really standardised internationally, other than for 'card holder present' i.e. in a shop, over the counter transactions.
Visa Debit is your only option for international debit acceptance, and that's only available from Halifax in Ireland at the moment.