As he sparred with Denis O'Brien this week in a phoney war, Sir Anthony O'Reilly made much of his experience on the global business stage.

"You have to realise that I was for 25 years the president of the HJ Heinz company and in America the whole issue of dissident shareholders is a constant theme in almost every board that I've ever been on," he said after IN&M's annual meeting this week.

"And I've been on the board of Mobil Oil and Bankers' Trust and the Washington Post and the HJ Heinz company, so I am somewhat a veteran of the wars in terms of dissident shareholders."

Sir Anthony went on to say he regarded all dissident shareholders as having a particular agenda of their own. "What it is I don't know, nor would I wish to attempt to estimate what that is."

If Sir Anthony's message was that he is more prepared than most for what is shaping up to be a bruising battle with O'Brien, his adversary has no shortage of cash or corporate experience.

After increasing his bank balance by €600 million last February in a refinancing of his Caribbean mobile group Digicel, it is a given that O'Brien has shown that he has powerful friends in the international capital markets. So too does Sir Anthony.

O'Brien's lieutenants say IN&M's governance is more suited to 1967 than 2007 but he has so far offered no public insight into his plans.

Some stock market observers divine that his intention is to buy out IN&M before selling off its units in Britain, South Africa, India and Australia.

This would enable him to cut the cost of his borrowings for a leveraged bid while gaining control of an Irish business which made operating profits last year of €94.4 million. Some also speculate that he might also use IN&M to buy out his interests in Communicorp, his personal media company which has radio interests in Ireland and eastern Europe.

However, such a strategy might run into opposition from competition regulators.

Still, O'Brien will face a formidable fightback from Sir Anthony in the event of a bid.

The group this week tightened up its rules on the disclosure of beneficial interests, a manoeuvre that enables it to more closely monitor any further accumulation of shares by O'Brien.

Though backed by only a slim margin of IN&M investors, the move illustrates the seriousness with which the group takes the challenge from O'Brien.

With great speed, the battle lines of this confrontation are taking shape.