O2 Ireland's chief financial officer, Mr Niall Norton, has resigned to take up a post at a much smaller Irish tech firm.

Mr Norton, who joined O2 Ireland in 1997, becomes chief financial officer at Openet Telecom, a fast growing Dublin-based software company. Openet supplies charging software to the telecoms industry and has been funded by Cross Atlantic Venture Partners and the venture arm of mobile firm Orange.

Former O2 Ireland chairman Mr Barry Maloney, a director of Benchmark Capital which has invested $20 million in Openet, is also a director of the Dublin tech firm. Openet generates annual revenues of $15 million and plans to float on the stock exchange within three years.

Mr Norton's exit comes at a sensitive time for O2 Ireland, which this week lost a potentially lucrative national roaming deal with Hutchison. The firm had been engaged in talks with Hutchison - a new entrant to the Irish market - until recently but lost out to a Vodafone bid.

The deal will enable Vodafone to generate additional revenue every time a Hutchison customer makes a call in an area where the firm does not have a network.

O2 will also have to negotiate with ComReg on the regulator's proposed ruling in the mobile phone sector that may prise open its network for use by so called "virtual operators".

An O2 spokeswoman said the firm was looking for a new chief financial officer and an appointment would be made shortly.

Ms Danuta Gray, O2 Ireland's chief executive, said Mr Norton had made a long-lasting commitment to the success of O2.