Allied Irish Bank (AIB) says that "technical issues" have led to problems with some online, mobile and branch transactions.
The bank took to Twitter at around 12:00 local time to warn about the problem, telling customers that they could still withdraw cash from ATMs and make purchases online and instore with their AIB cards.
Two hours later, most services were back up and running, although the AIB mobile banking was still out, with customers advised to use online services instead.