If you send an invoice (for services) to customers in other European countries from Ireland - a reference should be put on the invoice explaining that the transaction might be subject to the reverse charge basis of accounting for vat along with the customers VAT number.
The reason for the customers VAT number is to show that they are a valid business that falls under the business to business (B2B) place of supply rules.
We use the following reference:
Acc. To Art. 196 VAT directive supply of services might be subject to reverse charge mechanism
<Insert customers VAT Number>
From the Revenue website: "http://www.revenue.ie/en/tax/vat/leaflets/place-of-supply-of-services.html"
Where the service is supplied to a VAT registered customer in another Member State, the supplier must ascertain the VAT Number of the customer and that number, together with an indication that the reverse charge applies, must be included on the supplier’s invoice, along with the supplier's own VAT Number and details of the transaction. It is possible to verify the format of a VAT Number at the EU Commission website or whether a VAT Number is appropriate to a particular trader by contacting the VIMA Office at firstname.lastname@example.org.