His foul-mouthed outbursts and penchant for outrageous stunts have made Ryanair boss Michael O'Leary one of the most controversial figures in modern aviation -- but few would deny his abilities as a businessman.

O'Leary, 45, a former accountant, took a loss-making regional Irish airline and in little over a decade transformed it into Europe's biggest budget carrier, amassing a personal fortune approaching EUR500 million (USD$635 million).

On Thursday, with characteristic drama, he embarked on a drive to make his company even bigger, catching the market unawares with a plan to buy newly floated Irish rival Aer Lingus -- an airline he has publicly lambasted for years.

The move, if successful, would extend O'Leary's reach into US and other long-haul markets and increase overall passenger numbers to an annual 50 million.

A takeover would also potentially irritate two traditional targets of his legendary vitriol: the Irish government, which has 28 percent of Aer Lingus, and the airline's trade union.

Other O'Leary targets -- and there have been many -- have included the European Commission, which he dubbed "an evil empire", and politicians, whom he has dismissed as "loonies". His verdict on travel agents is unprintable.

Ryanair's aggressive advertising campaigns have included a poster of a nun whispering in the Pope's ear and a campaign branding British Airways "Expensive Bastards". O'Leary himself once dressed as the Pope to preach a "low-fares sermon" -- earning him the wrath of the Vatican and his own mother.

While O'Leary's detractors argue his tactics have done him few favors and won few friends either at home or abroad, his supporters hail him as the proverbial Irish underdog waging a heroic struggle against bigger rivals.

The man himself -- who prefers jeans and a rugby shirt to a business suit -- acknowledges his own irritation factor, once describing himself as "a jumped-up Paddy" and "a gobshite".

Contrary to the cultivated country bumpkin image he portrays, however, O'Leary hails from a wealthy farming family and received a public school education at the alma mater of celebrated Irish writer James Joyce.

In 2004, he generated bemused headlines in the Irish media when he bought a hackney plate for his personal Mercedes enabling it to be classified as a taxi so he could legally use Dublin bus lanes to bypass the city's notorious traffic jams.

O'Leary started working for Ryanair -- set up by Tony Ryan in the mid-1980s -- for a share of profits and set about revolutionizing it along the lines of Southwest Airlines of the United States, the world's original low-cost airline.

His Wal-Mart-style strategy of "pile it high and sell it cheap" has seen air travel stripped back to its essentials -- no meals, no sick bags -- and opened up the skies to a new generation of tourists flying to new destinations.

Speculation that marriage in 2004 -- and subsequent fatherhood -- would calm him down proved groundless.

He has shown few signs of seeking full-time retirement at his 250 acre farm in Mullingar, an hour's drive from Dublin, where he keeps a herd of prize-winning Aberdeen Angus cattle.