Denis O'Brien's bid to purchase Eircom's fixed line phone network is his most audacious move yet.

It is also deeply ironic because he made his fortune by taking on the former State monopoly. When his company, ESAT Telecom, was sold to British Telecom for £1.9 billion last January, Mr O'Brien gained about £231 million.

In a separate development yesterday, the Supreme Court overturned a High Court decision to award him £250,000 in a libel action taken against Irish Mirror newspaper. Although the original verdict was not affected, the court set aside the original award, stating it was disproportionately high.

Many of Mr O'Brien's close colleagues at ESAT - including its co-founder, Mr Mark Roden; a businessman, Mr Leslie Buckley; and a former secretary at the Department of the Taoiseach, Mr Padraig O hUiginn - also made significant sums on the BT deal.

Other beneficiaries included his father, Denis senior, founder of a horsefood supplement company, Plusvital Ltd, which is based in Clonee, Co Meath.

When the telecoms market was deregulated in the 1990s and the monopoly enjoyed by Eircom challenged for the first time, ESAT emerged as its most significant competitor.

The company Mr O'Brien founded in 1991 has yet to turn a profit, although analysts believe its forward-looking position is strong.

His most significant coup was securing the second mobile phone licence in 1995 for ESAT's subsidiary, Digifone. That company, which recorded losses last year of £27.49 million on sales worth £222.15 million, now has over 700,000 subscribers.

O'Brien targeted corporate customers first, though breaking into the market was difficult initially. He took frequent court actions as the telecoms business opened up.

Described as a "doer" and "ideas man", Mr O'Brien is chairman of the Dublin radio station, 98FM, and owns 71 per cent of an electricity company, ePower. It claims to have secured 83 industrial customers since the power market was partially deregulated in February and has already taken one court action against the Commissioner for Electricity Regulation.

In addition, Mr O'Brien is a significant investor in a number of start-up technology firms - he has signalled an intention to invest about £30 million in Irish Internet firms this year and next.

Mr O'Brien (42) is from the Ballsbridge area of south Dublin. A former assistant manager at Trinity Bank, a small merchant bank, he also worked as personal assistant to the businessman, Dr Tony Ryan, who founded the budget airline, Ryanair.

Mr O'Brien, who studied at UCD and Boston College, is married with one child. For tax reasons he lives in Portugal, where he owns a major golf property at Quinta do Lago, which he purchased in 1998 for £25 million.