Best PAYG mobile for ROI

Can anyone tell me the best option for a Pay as you go Mobile for occasional use in ROI.

I want to use it mainly for internet use (plus a few calls). I plan to top up (online if possible) prior to a trip. We have discovered that this could be less expensive than our roaming charges.

Plus we have found that viber is great if you have internet coverage.

Any help will be gratefully received....

I could soon get an unlocked phone.

So Vodafone has better coverage. Thanks, that's good to know.

I brought a guide from car phone warehouse in Kilkenny at the weekend. Will have a look at Vodafone options.

Thanks again wayfarer