Irl Ltd Billing a UK Company - Vat or not?

If they are VAT registered and supply you with their vat number you dont charge them VAT (Same with all EU Customers)
Or as the revenue put it
- the customer is registered for VAT in that other EU Member State,
- the customer’s VAT registration number (including country prefix) is obtained and
retained in the supplier’s records,
- this number, together with the supplier ’s VAT registration number, is quoted on the sales
invoice,
- the goods are dispatched or transported to that other EU Member State.

Sorry I half assed read your post, Services are slightly different I've never dealt with your situation in the 'real world' so i'll leave it to someone who has, but here's what the revenue have to say on the matter:

Fourth Schedule” (or “received”) services
4.8 A number of services, all of which are listed in the Fourth Schedule to the VAT Act, 1972 (as amended), are deemed to be supplied in the place where the customer is located, provided that they are supplied for business purposes within the EU, or for any purpose outside the EU. Supplies of these services to private individuals within the EU are taxable under the general rule (see paragraph 4.4).

The services affected are:-

(a) Transfers and assignments of copyrights, patents, licences, trade marks and similar rights;

(b) hiring out of movable goods other than means of transport;

(c) advertising services;

(d) services of consultants, engineers, consultancy bureaux, lawyers, accountants and other similar services, data processing and provision of information (but excluding services connected with immovable goods);

(e) telecommunications services;

(f) acceptance of any obligation to refrain from pursuing or exercising in whole or in part, any business activity or any such rights as are referred to in paragraph (a);

(g) banking, financial and insurance services (including re-insurance, but not including the provision of safe deposit facilities);

(h) the provision of staff;

(i) the services of agents who act in the name and for the account of a principal when procuring for him or her any services specified above.

http://www.revenue.ie/index.htm?/leaflets/vatguide_chapt5.htm