With cards issued by the more ‘popular’ banks (AIB or BOI etc) you can top up your Paypal account from your credit card account – just go into your online banking & choose your credit card as the account to be debited instead of your current account & use the Paypal Beneficiary details from the site. I don’t know if you can use the likes of MBNA, though if the facility for bank transfer is available you should be able to also.
It’s just as easy to, as pudds has said, link your credit or debit card to your Paypal account. Just go to My Account, then Profile, Financial Information, then Credit/Debit Cards. By doing this whenever you make a purchase using Paypal, whatever funds are in your Paypal account are used first & the remainder will be deducted from your credit card. The transaction would be treated as a purchase, instead of a cash advance which would be the case if you did it by bank transfer from your credit card.
I think it's best to do it by linking your credit card, as Paypal have a nasty habit of 'locking' people's accounts if they don't like the look of a site that they may have made a purchase from, so if you actually have no funds in there to begin with at least you can potentially lose nothing if you can't persuade them to unlock it.