Denis O'Brien (41), once a personal assistant to Mr Tony Ryan, founder of GPA, is best known for founding Esat Telecom in 1991 as well as for his involvement in the Dublin independent radio station, 98FM.

Mr O'Brien had to fight very hard to break into the telecoms market at a time when it was not deregulated. The first sector to be liberalised was the corporate market, which he targeted aggressively.

He has always surrounded himself with good advisers, both legal and financial, and has been praised for the tenacious way he loosened the grip of Eircom (then Telecom Eireann) on the telecommunications market. He made complaints to the EU and exploited the law to his advantage wherever possible.

The prize for him and probably his own greatest personal triumph was winning the second mobile phone licence in 1995. Although controversial - there was much surprise that the Irish company had beaten off stiff competition from big international players, including Motorola - Esat Digifone proved a strong competitor in the market.

Mr O'Brien, who is married and whose first child is due in January, has always said he is not a seller. He has some days to unveil a strategy of resistance to the current offer, or to find another buyer - and he should never be written off. When he heard about the bid, he is said to have told associates that he wasn't worried, that the offer would need to be a lot higher to succeed. However, he certainly realises he has a responsibility to recommend acceptance of any offer that is generally thought to value the company properly.

As for the battle ahead, he is said to have told associates that he has no fears: "I spent long enough fighting Telecom Eireann to create Esat; I'm used to it," he said, according to one source.

Through various investments, he is now a multi-millionaire. His investment in Esat is currently worth in excess of £180 million (€228.55 million) and his stake in Versatel, a Dutch telecoms company, is said to be worth more than £100 million.

Last year, he bought the Quinta do Lago leisure complex in Portugal.