What are the five most chilling words in the English language? For millions of people, it's the phrase "cannot connect to the internet" on their computer screens.

In a nation that was slow to embrace the joys of email and the web, Ireland's recent rapid uptake of broadband has propelled us out of the bottom of the world league.

Look how much we have come to depend on the web: entertainment, grocery shopping, banking, booking flights, making appointments, storing photos, finding lost friends, buying books, music, gifts, etc.

This week RTE launched a new service that means you may never need to switch on your TV again. The RTE Player stores many of the most popular shows broadcast on RTE1/RTE2 and enables you to watch them online at any time up to 21 days after transmission.

Irish internet expert Michele Neylon, who runs leading website hosting company Blacknight, says: "Until a couple of years ago, many businesses and individuals were using the internet -- but not relying on it. But that has changed dramatically. Now it's practically impossible for a lot of businesses to function without the internet."

But what if your internet connection regularly went down for hours or became so slow as to be unusable? Maybe you can feel the withdrawal symptoms already.

That's the nightmare scenario painted this week by US thinktank Nemertes Research, which predicted that skyrocketing demand for internet capacity would exceed supply by as early as next year.

The technical term is a "brownout", where the internet's pipes become so clogged that millions of users would be effectively cut off. Stuck in a traffic jam, email, web pages or videos would be unable to get to your computer for minutes or maybe hours.

"With more people working or looking for work from home, or using their PCs more for cheap entertainment, demand could double in 2009," said Nemertes analyst Ted Ritter. "At best, we see the [economic] slowdown delaying the [internet] fractures for maybe a year."

The problem lies with the strain on the internet created by new media such as YouTube videos. Emails, web pages and even photos consume relatively little bandwidth, a measure of the amount of information flowing through the internet's pipes.

Video, on the other hand, gobbles bandwidth, leaving much less capacity for the other traffic on the net.

What is certain is that consumption is rising. Rough estimates of internet traffic in Ireland alone show a fivefold increase since 2004 and a doubling in just the last year.

YouTube alone now generates more traffic per month than was carried across the internet in the whole year of 2000. Some estimates believe internet traffic will increase 100-fold by 2018.

Of course, YouTube is currently the 800lb gorilla of the internet but it's one of only dozens of sites offering video on demand. The BBC's equivalent of the RTE Player caused huge outcry from internet providers in Britain when it launched last year because it consumed so much bandwidth. Downloading Hollywood movies via iTunes is already possible in the US and UK and will no doubt arrive here soon.

Yet not everyone is convinced of Nemertes' pessimistic analysis, and internet history is rich with predictions of brownouts that never happened.

The internet is remarkably good at routing around problems, something that stems from its origins as a military communications network in the 1960s.

There is no single point of failure.

Say a construction worker in Ballydehob severs an important underground internet cable. Only a few people locally may be temporarily affected while the rest of Ireland will be blissfully unaware as the net routes traffic via other links.

"I'm very sceptical, I don't think there's going to be major issues," says Michele Neylon, explaining that most responsible internet companies are always planning ahead for expansion, ensuring they have extra bandwidth on tap to switch on at a moment's notice.

"We have plenty of headroom. We have up to 10 times the capacity of what we currently use."

Neylon also points out there are ways for internet service providers (such as Eircom, BT, etc) to avoid bottlenecks and clogged pipes. Technology such as caching, where copies of popular videos are stored in Ireland rather than on some US server, means our international net links suffer less strain.

Paul Gunning, who researches technology to boost internet speeds at BT Ireland, is blunt: "There's lot of capacity, almost infinite in some cases. I don't see it as a problem and it's my job as a researcher to see that we don't get to that position (of clogged pipes)."

Like most internet providers, BT recognises the challenge posed by the explosion of online video. Paul Gunning is part of a team collaborating with the University College Cork to find ways to squeeze more information more efficiently through the internet.

Their research hopes to increase the capacity of their network -- using the same cables -- by up to 100 times within five years.

"If you had another huge increase in demand for bandwidth, it could even be brought forward," says Dr Gunning.

So capacity issues are unlikely to bring the internet to its knees, but maybe cyberterrorists have a better chance.

Hollywood has found rich pickings in hackers since Matthew Broderick starred in 1983's WarGames as a teen who nearly triggers a nuclear war while hacking into US military computers.

Two years ago Die Hard 4.0, starring Bruce Willis, featured hackers who took control of the stock market, transportation, electricity and military computers.

Overacted and overblown it may be, but governments fear the plot could be frighteningly close to reality in the future.

Real-world hackers have graduated from creating viruses that infect Windows PCs but do little damage. Now criminal gangs can silently assemble vast "armies" of home computers under their control. For now, many of those armies, known as botnets, may be used to transmit spam or attempt to steal banking passwords. Some have been marshalled to attack high-profile websites in an effort to extract a ransom.

Already, botnets have been used for political purposes. China has been accused of being behind attacks on Tibetan websites.

Russia was fingered for an electronic assault that crippled many of Estonia's key websites in 2007.

But governments worry that botnets could be turned on sensitive targets such as those attacked in Die Hard 4.0.

NATO takes the threat so seriously that it has set up a base in Estonia to coordinate its defences against cyber-war. The Cooperative Cyber Defence Centre of Excellence in Tallinn houses security specialists and researchers working furiously to anticipate cyberterrorism and devise counter-strategies.

Suddenly, the prospect of a slowdown in your YouTube viewing seems small beer compared to the scenario of hackers holding countries to ransom.

We may not be able to do anything about the bigger picture but we can take care of things closer to home, such as updating our antivirus software and downloading security patches regularly.

Net expert Michele Neylon warns too of one the greatest online problems, phishing, where fake emails lead you to input names and passwords into fake websites.

"Phishing is a huge problem, enormous, absolutely gigantic," he says. "So much of people's stuff is online. If a person gets into somebody's gmail account, they might get most of the keys to the kingdom -- they could take over a person's life."

With that scary thought, some may feel that a broken internet connection wouldn't be so bad after all.