Under the new 'free float' methodology, which assigns weightings to Iseq companies on the basis of how many of their issued shares are available for sale, AIB and CRH alone will now account for a third of the market.

But in a week when financial and construction stocks got disproportionately punished in the Irish market over the latest quarter-point rise in interest rates, the news that economically-sensitive banks and a construction materials firm will count for yet more in the Iseq index raised the prospect of greater market volatility under current conditions.

"There's a logic to that playing out, " said Stuart Draper, head of research at Dolmen stockbrokers. "As the weighting becomes more financial services-driven, you could see more volatility."

Meanwhile, Bernard McAlinden, investment strategy analyst at NCB stockbrokers, said the international investors who drive the market here were in a mood to punish rapidgrowth peripheral eurozone economies such as Ireland anyway.

"International investors are not that interested in relative weighting, " he said.

"They're in here on a theme or a story, like the Celtic Tiger, and when that story changes, they can sell and leave the market in the doldrums."

He said financial stocks were especially vulnerable because "that's where the neurosis is at the moment".

Indeed, even the latest record results by Bank of Ireland and Anglo Irish Bank in May were met with sell-offs as neither surprised investors with an upgrade.

"There was a pattern of expectations where for a long time banks not only delivered results, but also surprise upgrades, " said Draper. "Now the pendulum has swung too much the other way because banks aren't delivering upgrades in line with current market expectations."