Carlow and the Congo may not, at first glance, appear to have much in common. But the two came together this week when Tullow Oil bought Energy Africa in a $500 million deal that makes it the largest exploration company in the West Africa region.

The deal, which doubles Tullow's projected cashflow, reserves and production, is expected to transform the Irish company, catapulting it up the league of independent producers.

To Tullow's existing oil and gas operations in countries such as the Ivory Coast, Gabon and Cameroon, it adds interests ranging from Morocco to South Africa and from Egypt to the Congo.

"Energy Africa has the ability to be a transforming deal. It gives them a scale they've never had before," says Davy analyst Mr Job Langbroek. "This puts them on more investor radar screens and gives them cashflow so they can look to the next deal."

The shares, suspended since March pending the announcement of the deal, have risen by 18.5 per cent since they resumed trading on Tuesday. At last night's close, the company was valued at around €570 million.

So how has Tullow made it from humble beginnings in Carlow to its current status as the major player in an up and coming region in its sector when so many other Irish exploration companies have fallen by the wayside or remain consigned to the margins?

Astute deal-making, rather than drilling success, appears to be the answer.

Founded by its current chief executive Mr Aidan Heavey with the backing of his employers at Tullow Engineering, the Nolan family, the company began life with just one major project - in West Africa.

"Back in those days, everyone was looking at setting up businesses in the Celtic Sea," Mr Heavey says. "Everybody wanted to set up oil companies, it was the flavour of the month, like the dotcom business four or five years ago."

But rather than follow the crowd, Mr Heavey took a tip from a banker at Barclays and acquired a licence, partly funded by the World Bank, to develop some old gas fields in Senegal.

"That was where the company really began, all the expertise of the company started off there," Mr Heavey says.

In 1988, the company listed on the stock market, going on to become involved onshore UK as well as in a variety of other locations such as Yemen, Syria, Italy, Spain, the Czech Republic and Pakistan.

"We've been in quite a few places," says Mr Heavey. "We made money out of most of them. We made $10 million out of Yemen, money out of Syria and Czech Republic. A lot of these assets, we would have been in and out of them quite quickly. When they didn't show the upside we felt was needed, we sold them on."

But, in reality, Tullow remained just another high-risk Irish exploration stock with interests in a handful of far-flung sun spots.

What really propelled the company into a different league was its acquisition four years ago of £200 million sterling of gas producing assets in the North Sea from BP. The deal doubled the size of Tullow and increased its production output fivefold. And, like many good deals, it all began on the golf course.

"We had targeted the southern basin in the North Sea as a place we would like to get involved in and one in which we saw upside," Mr Heavey says, adding Tullow had contracted a British bank to look out for £50 million deals in the area.

Over a game of golf in Druid's Glen, one of the British bankers mentioned the possibility of BP selling all its North Sea gas assets as a condition of EU approval of its merger with Atlantic Richfield. "We went to see BP the next day," Mr Heavey says. "We moved fast."

The deal not only added to Tullow's size and took it into the UK North Sea at a good price, it changed the perception of the company, bringing it to the attention of a wider investment audience.

Mr Heavey believes the Energy Africa acquisition is an even better one for Tullow, offering greater potential. But the deal wasn't done overnight and drew on Tullow's long history in West Africa for its success.

According to Mr Heavey, Energy Africa was sitting on the Johannesburg Stock Exchange, ignored after a hostile bid was blocked by minority shareholders in 2002.

"Basically, it was viewed as an asset you couldn't acquire, which created a challenge which we went after and succeeded," he says.

To secure the deal, Tullow approached the many individual shareholders in the company, most of whom were prominent African businessmen - including former OPEC chairman Mr Sam Doussou - to win their support. It also went to all of the governments in the countries where Energy Africa has interests, to make sure a deal with Tullow would be well-received.

"It was an Irish approach. We went in and negotiated and worked out a deal, a solution with the existing shareholders of Energy Africa," he says.

Although Tullow is in many ways a UK company - it has re-registered in Britain, the bulk of its shareholders are based there and the bulk of trading in its shares takes place in London - it remains proud of its Irish identity which it has used to its advantage in Africa in particular.

"We still travel as the Irish oil company. It's on all our literature and brochures and baseball hats," says Mr Heavey. "The Irish are very welcome in places like West Africa. You get a very good reception, you don't bring any baggage with you - it's a good calling card." So where next for the company?

Its first challenge is to successfully cope with the acquisition of Energy Africa, a company that on certain measures is bigger than Tullow itself.

Analysts say that the African company has some very interesting exploration acreage in Morocco and Mauritania, in addition to its production interests in Uganda, Congo and Equatorial Guinea. Tullow's other two main areas of interest - UK offshore and the Indian sub-continent - will also remain a focus.

But, while the drilling will continue, deal-making is likely to remain as important to Tullow as the search for black gold.

The company's growing size, allied to cashflow estimated at around £170 million sterling annually, means acquisitions should be easier to come by, as Tullow continues to try and exploit the opportunities thrown up by the gap between the independent producers and the super majors. But timing will remain key.

"When you're a small company, if you're drilling an exploration that requires a lot of luck. But you can't grow a business just on luck, so you have to try and be ahead of the posse," Mr Heavey says. "We are making sure we are ahead of the posse in relation to deals."