Somebody may have the answer!

If I were to use my Portuguese Bank card from our UK address / email address it obviously knows we are in the UK and puts a charge on, fair enough.

However if I was in Ireland (Southern) on holiday and then decided to book another holiday whilst we there using the accommodation's WiFi which will have an ie. extension would that then not encounter a charge, much like just going into a shop in Ireland and paying with a Portuguese card.

Are you sure it's the location you're in when you use the card for an online purchase that triggers a charge?

That doesn't seem very logical to me. I'd have thought it was more to do with where the payment comes from and goes to - so if you are buying something from a Eurozone entity, paying with a PT bank card should eliminate conversion, and commission charges etc but if you use it to buy something from a UK one, it'll incur a currency conversion charge, commission and stamp duty on both of those charges.

Zero logic from OP.
What do your T&C for abroad usage says, all info you need is there.

Yes you are correct, a major bearing on the purchase is the currency it is offered in.

For instance if I book with say EasyJet or Booking.com from the UK the price will be displayed in Sterling.

So answering my own question really if I were doing a search whilst in Ireland the prices would be displayed in Euros and would not incur any conversion charge.

I think we got there 😊

Red Eric is right. It all depends which card you are using. Easyjet will show you Euro price if you are in Europe. If you want to pay with a UK card make sure to change the cost into GB currency before you make the payment.I forgot to do that once and I was charge a conversion fee. If you use booking.com the price shown is only estimated and you will pay after your stay in the currency where the hotel is located.

When I had to pay outstanding income tax in Germany - to avoid conversion charges (£ to €) I transferred the money from our current account in Portugal.

Postscript.

Just been into Booking.com and you can change your currency to Euros so that leads me to believe I could indeed book from the UK and use my Portuguese Card with no charges.

Edit. We still have a chunk of money sitting in our bank account in Portugal, hence the question. And yes I do know how to transfer it back!

In my experience, it is the origin of the flight (the outbound in the case of a return) that determines the currency charged by the airline not where it is booked. Some airlines will offer to convert the final price to your card currency (if different) but this may not be at an advantageous rate for the traveller.

Indeed . check out Ryanair's "Dynamic Currency" scam, for example, and the way it's automatically applied unless you opt out (not an option that's presented to you unless you know what's going on and how to avoid it).

Most vendors use the currency of their country; when you change the currency display in (for instance) booking.com, it just converts the number on the screen.
Your actual charge will be in the local currency of the country of the accommodation, and exchange charges may apply.
Booking.com gets 10% from them.
Unless you go to the hotel's own website and book from them directly, like I do, because 10% is way too much and they're a monopoly and it's a racket.
Where you log in has no bearing on the matter.

Chislenko
Talking about keeping your Portuguese bank account- I don't know who you are banking with but our bank Santander Totto posted (normal post without having to sign for) a new bank card (1 month before it expired) to our holiday home in Lagos. Luckily our neighbour intercepted it. When we arrived 3 months later we were able to use the card straight away without the need to have it validated. Not a good system!!

I was told by the owner of a five star hotel in New Zealand that booking.com gets 15 percent of room rate. Way too much

Booking,com bought / merged with its competition, so it has a stranglehold on hotel business all over the world.
I don't think it's even legal.
Hotels and guest houses have huge capital costs and operating expenses, and I think it's outrageous that a computer in California gets that big a slice of the gross take.

So find your hotel on Booking.com, then google the place and reserve your room on their own site or even (gasp) over the phone. That will take 2 minutes extra.
Sorry, off topic rant.

Nata, don't know if by planning or luck when we opened our account years back we opted for all mail to be sent to the UK. I think card validation is now a thing of the past with most banks, the only form of validation nowadays appears to be that the first time you use a new card it has to go in a card machine and have your pin number inserted. It won't "tap" until then.

However from a security point of view I think you are right to be concerned as if it did fall into the wrong hands there would be little to stop them doing internet / phone transactions.

Yes, agree with your last part, however we usually book a house / apartment and getting contact info is quite difficult as they are usually only marketed through an agency like booking / cottages / Sykes etc.

Just google the address, or the hotel name+ address, or their phone number.
9 times out of 10 that will work; of course if the hotel has no independent booking system then they just have to pay the dot com billionaires their piece of the business.