I would ask the customer if they can claim back Irish VAT, if they can you have a choice and can charge them VAT. If they can't (or you think it's more appropriate depending on the service you provide) set up the invoice as a 4th Schedule service whereby the customer accounts for the VAT.
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It's a bit of a grey area IMO and the Revenue 4th Schedule leaflet lists telecommunication as a 4th schedule service.
I think either way you need their VAT number so call their accounts department and discuss with them before you send the invoice. Otherwise payment might be delayed if they're not happy with the invoice you send them.