If you are travelling to a juristiction with a different currency, you will be charged by someone, somewhere for a currency conversion. You cant avoid paying a bank,
even if you consider this a "rip-off"
Laser does not work abroad for POS purchases, but can be used in ATM if it is a Maestro or Cirrus card also.
If you get cash converted in Ireland, you will be charged a fee via the conversion rate
If you withdraw cash using your CC or ATM card in London , you will be charged a fee and be subject to the conversion rate they apply
If you use your CC for purchases, you will be subject to the conversion rate they apply
You need to find out what these fees and conversion rates are, and decide which suits you.
I would suggest asking your card provider for this information.
I think the safest and cheapest way to spend money abroad is to use CC for all purchases where possible and use cash for small items like publoic transport / taxis / snacks etc.