I guess I should have known this was a problem. However, it was all lastminute.com - couldn't be helped as I had little in the way of notice.
If you don't have an account with a bank, then either they won't accommodate fx transactions OR (Ulster Bank) will only spare a couple of hundred for non-account holders.
My bank (PTSB in Galway) - said I'd missed the run for this week (even though it's due to be delivered on Friday) - and that it would be available for collection based on an order today. They then checked what they *could* give me based on what they had in stock - and said that between the 2x Galway branches, they couldn't give me a single dollar!
I got a bad rate - but just to get the problem solved, was happy enough simply to get sorted via An Post.