Now, however, fuelled by the public's increasing concern over climate change and high fossil fuel prices, an increasing number of businesses and investors are talking of a green energy boom.

Some are big established companies, such as NTR, which have fundamentally changed their business model to cash-in on the boom. Others, such as Irish businessman Simon Dick, are dotcom survivors who see a chance to make their second fortune.

According to Dick, founder of location technology firm Mapflow, the attractions of the renewable energy sector are obvious for entrepreneurs.

"Renewables represent a very exciting shift in a very big market. Climate change and security of supply are major issues and people are only starting to realise it."

Dick now heads Clearpower, a renewable energy firm that hopes to capture a significant share of the Irish heating market using its wood-fuelled boilers. He is also an investor in its sister company, Clearpower Technology, which is developing the WaveBob, a wave energy converter invented by his father, William.

The rapid growth of the renewables sector occasionally reminds him of the dotcom boom but he feels the rewards are much greater this time.

"The size of the energy market is colossal compared to the software market. There will be big businesses made out of this. The Irish heating fuel market, which represents around 40% of total energy consumption, is worth around 3bn to 4bn a year, " he said.

Clearpower's current focus is on the corporate sector and it offers outsourced energy services aimed primarily at apartment owners. Under this system, Clearpower looks after the fuel, equipment and operation of a central boiler at the core of the apartment development and is paid monthly by the management firm.

Dick hopes his company can grow organically by increasing wood energy's share of the overall heating market.

"If Ireland can get to a situation where 10% of its heat energy came from wood . . . our stated target and the EU average . . . that would create a market worth up to 400m a year.

And that's just fuel, " he said.

Asked whether the boom in renewables could lead to a market bubble similar to the dotcom boom, which ended in a stock market crash wiping 78% off the value of New York's Nasdaq in 18 months, Dick was circumspect.

"In any rapid-growth market, you will get companies that won't be there in two years and you'll get chancers coming in.

But the energy market moves more slowly than the tech sector and you can't make a quick buck in a few months, " he said.

"I'd say that the people who will get stung will be retail investors but I think there'll be less of an opportunity for that to happen this time."

Dotcom survivor Todd Jones, executive vice president of business operations for carbon offsetting firm AgCert, is also confident about the longterm viability of renewables.

Jones, who worked in a company developing virtual reality headsets during the dotcom era, said: "I rode the dotcom bubble up and down and there are a couple of differences in my mind about the two. Firstly, this sector is driven by real products, whereas the dotcom boom was driven by hype, dreams and vapourware.

"Secondly, in the dotcom era, consumers supported the stocks and some commercial elements regarding website transactions but you didn't have their full backing. But this time, I think you have solid consumer support: there's a swelling of people who want to see this occur and are willing to pay a premium for renewables."

AgCert, which was established in 2002 and is listed on the London Stock Exchange, generates and sells carbon emission reductions, many of which are produced by capturing the methane produced by livestock and burning it off.

The company has grown rapidly in recent years and the number of emission reductions sought from it increased to 134,508 tonnes in 2006 from 1,897 tonnes the previous year.

However, the company has been forced to revise its longterm strategy after missing its targets. In recent months, the whole carbon reduction sector has come under fire, with British low-cost airline Easyjet accusing dealers of being "snake oil sellers" who charged excess commission for unverifiable reductions. There is also increasing concern that one of the main carbon markets, the voluntary market, could be prone to fraud as it is unregulated.

Jones admitted that AgCert was concerned by the negative publicity.

"We're becoming more aware of the sceptics in the market and all we can do is retain the integrity of our firm and retain the integrity of our market. It's important to note that companies like AgCert don't enter into voluntary market arrangements without third-party certification. We don't want to be in a place where our brand gets sullied."

One Irish renewable company that was recently in the news for the wrong reasons was tidal energy firm OpenHydro. One of its directors, Herbert Williams, was forced to resign when it emerged he had been convicted over the construction and design of catamaran for Colombian cocaine dealers.

However, the company's chief executive, James Ives, said that the incident would have no long-term effect on the firm's development.

"It had absolutely nothing to do with the technology or the company, " he said.

OpenHydro develops underwater tidal turbines and Ives said the company was considering a stock market flotation in the future.

"We would look to get some liquidity in the shares, " he said.

Ives said he was confident that the renewable sector would develop into a long-term growth industry and not be another stock market bubble.

"The difference compared with the dotcom boom is there's such an interest and urgent demand for this technology.

Oil prices have increased dramatically, as have environmental concerns."

The increasing interest in renewable energy has led some established firms to amend their plans. NTR now focuses on renewable energy with subsidiaries such as the country's leading windfarm operator Airtricity and bioenergy specialist Bioverda.

"These are still quite early stages for these industries but I would say the prospects of how they will develop are strong, " said NTR's group development manager, Ian Simington. "If you consider both the EU's renewables targets and the remaining lifespan of our conventional energy sources, you would have to conclude that there is room for some industries to grow threeor four-fold."

Simington said there was little chance that investors would lose out as they did during the dotcom boom.

"While there's a lot of investment money out there that thinks the investment story is good, they don't want to invest in individual projects but credible players in each sector."

He declined to comment on whether NTR would consider a stock market listing in the near future to fund the development of its businesses.

One of the main beneficiaries of the current boom in renewables has been the green investment fund. For instance, KBC Asset Management's alternative energy fund achieved 24.1% growth in the 12 months to the end of May while Dolmen's Green Effects fund has seen an 86% return over the past three years.

Despite this impressive performance, one investment manager, Graham Brooks, a director of F and C Ireland, said the renewables sector was still quite unstable from an investment point of view.

"Early last year, for instance, there was a bubble in biofuel because the oil price rocketed due to the Israeli conflict and people piled into biofuel stocks.

Then oil prices fell and the stocks fell too, " he said.

He said many biofuel stocks fell by an average of 30% in the second half of last year. He added, however, that there were stocks that represented reasonable value for investors "if you get in before other people see them".

Brooks is responsible for Friends First's Stewardship fund, around 10% of which consists of green energy stocks, and favours solar energy over wind and biofuels.

"The technology for solar has improved significantly over the past five years. They now don't need sun to create energy, they can do it from light, " he explained.

He said it was important for investors considering green energy stocks to research their options and not to buy when the oil price increased.

"When the oil price shoots up is not the time to invest in renewable energy stocks."