I've checked here on my end and haven't found any cases regarding the reconnection issue with the Bank of Ireland. Meanwhile, I can provide some fixes to get your banking back up and running again.
First, please log out and then log back in to your QuickBooks Online (QBO) account to refresh the system. Then, access your online banking again.
If it doesn't work, you can use a private window or an incognito when logging in to your account. Unusual behavior, like being unable to reconnect in QBO, can be caused by the piled-up cache in the system.
You can use these keyboard shortcuts depending on the browser you're using:
Ctrl+ Shift + N for Google Chrome .
Ctrl + Shift + P for Mozilla Firefox and Microsoft Edge .
Command + Shift + N for Safari .
Then, reconnect or access your online banking again. If it works this time, let's switch back to the regular browser and clear its cache & historical data. This helps to restore the default setup of the browser and refresh the system.
In case the private browsing doesn't work, you can another supported web engine to get the best and most secure experience with the program.
Also, you can reach out to your bank to verify if there are changes on their end. This can also be the reason why you're unable to reconnect your bank.