A copy of the invoice must be kept to show the figures that were adopted.
For conversion purposes you should use the Central Bank rates which are published in the daily newspapers at the time of supply.
It is possible to agree an alternative rate with the Revenue i.e. a calendar month exchange system. Please note the agreed method must then be used for all of your foreign currency transactions.
Your Northern Ireland client may prefer to agree a price in Sterling and pay you in Sterling, so that they are not exposed to exchange rate fluctuations. If you want to facilitate your client, you might agree a Sterling equivalent with your client, either on the invoice or as part of a separate contract or agreement. The disadvantage of this is that you then assume the exposure to exchange rate fluctuation. Some firms will state on their invoices that, should client companies wish to settle the invoice in Sterling, they should contact their accounts department on the day of settlement to agree a suitable rate of exchange on that day.
Do not forget to consider your own circumstances and whether or not it would suit you to receive Sterling at a certain point in time.