I haven't heard of foreign brokers deducting tax from share sales. They normally deduct taxes from dividends, but not from share sales. So Irish CGT at 20% of the gain (less your annual allowance of €1,270) will apply.
If you bought us shares through a us broker you should have filled out a w-8 form and s supplemental w-8 form.
If you haven't they can withohold 28% of dividends and share sale proceeds until w-8 submitted.
This form confirms you are a non us resident.
It is a form of withholding tax in essence.
Just to be clear the relevant US form is a W-8BEN. This is a declaration of non US residency which allows the broker to avoid deducting witholding taxes. Of course Irish taxes (e.g. CGT) will need to be declared and paid. I'm not sure what, if anything, is the equivalent UK version of this form. Best to check with UK Inland Revenue.
Don't forget when your calculating the rate of tax to pay you will need to convert the UK or USD equivilant to Euro both at the rate when you bough and sold the shares. The revenue have a fixed rate per year they will give you to calculate that amount or you can take the rate exactly on the day the trade was made. I usually work both figures and see which one comes out the best for me.