If they look for a vat number give them your PPS number but put IE in front of it.
If they say they can't confirm it tell them you just applied for it
Alternatively tell them you don't have to register for VAT until you sell Eur75,000 worth of goods in any one year. And as you're starting to trade it'll be a while before you get to that level.
http://www.revenue.ie/en/tax/vat/gui....html#section2