You would have to add the NI account as an international beneficiary. The set-up takes a couple of days as you have to get a code by post to complete the set-up. But once set-up, it's there for future re-use.
There is a charge for sending a payment to a non-EUR currency area. For GBP, I think it's a percentage of the transaction value, but BOI can confirm that for you.
If you go to a branch, it would be, for the first transfer, a day or two quicker - the rate and the charge would be similar. For subsequent transfers, using the browser would be more convenient.