The Republic of Ireland uses euro. Northern Ireland is part of the United Kingdom and uses pound sterling. A card may make the crossing feel seamless, but the currency conversion still happens.
Three road signs for the wallet
One island is not one currency area
Dublin, Cork, Galway and Donegal use euro. Belfast, Derry-Londonderry and the Causeway Coast use pound sterling. The geographical word Ireland can mean the island, while payment rules depend on the jurisdiction.
Cards hide the border, not the conversion
A Visa or Mastercard may work on both sides, but a EUR purchase and a GBP purchase reach your statement differently. A card with no foreign-transaction fee keeps the crossing simpler than buying a large pile of sterling for one day.
Do not assume shops exchange at the market rate
A business near the border may choose to accept the other currency, but it does not have to and may use its own rate or return change in local cash. Pay in the currency displayed whenever possible.
Use the local-currency button
If a terminal in Belfast offers EUR or GBP, choose GBP. If a terminal in Donegal offers your home currency or EUR, choose EUR. This normally leaves conversion to your own card network rather than the merchant's dynamic currency conversion.
Keep both reserves small and recognisable
Separate EUR and GBP notes in different wallet sections. A few small sterling notes are enough for a short Northern Ireland visit; unused euro remains useful throughout the euro area.