Three has been up to rather shady tactics recently and reckons it has found a loophole around the EU's upcoming abolition of roaming charges. Users on its "All you eat plans" would get as little as 1GB while roaming in the EU.
Although Three Ireland does not permit tethering on its prepay phone plan either, they don't actually enforce it (unlike Three UK) or charge users who do it. This is probably what is catching out the unlucky ones on bill pay plans as they probably figure it'll be the same. On the other hand, the "All you can eat" data expires 28 days after the €20 top-up, after which a €1.01/MB charge occurs whether the user tethers or uses data on the handset.
Three's 500MB prepay add-on has been broken for over a week now despite saying "We know about it and are working hard to fix it." The only way for a prepay customer to avoid paying €1.01/MB is to top-up by €20 every 28 days for AYCE or turn off data. A few app updates or photo sync uploads (e.g. 50+MB) is enough to wipe out one's prepay call credit without a data bundle.
Unlike other mobile providers in Ireland, Three does not issue any warnings that a bundle is about to expire, which also catches people out. For comparison, the network I'm on (Meteor) sends me a reminder a day before my bundle expires. Even if I forget and try accessing data, it displays a landing page asking if I would like to buy a new data add-on or to go online using out-of-bundle data.
Shortly after they bought out the O2 Network, they hiked all their price plans by around €5, increased their out-of-bundle call and data charges and changed from per second billing to per minute billing, rounded up. These price increases are likely one factor that resulted in the EU commission blocking Three UK's attempt to buy out O2 UK.