iPhone tethering Add-ons

The iPhone Internet Tethering Add On is available to new and existing O2 bill pay customers who are using an iPhone 3G or above, and are connected to an O2 Advance price plan, an iPhone Price plan or to a business price plan for Smart phone customers.

If your price plan does not include any data then a 1c per KB charge applies.
If your price plan has unlimited mobile internet browsing this cannot be used for tethering and a 1c per KB charge applies.
If your price plan has 1GB of data included then an additional data usage charge of 2c per MB applies after the 1GB has been used. For further details check your price plan page on O2.ie