'I don't know what a tracker mortgage is." Do these words sound familiar? Even if you never caught the Irish Financial Services Regulatory Authority's television ad in which they originally appeared, you've probably heard the catchphrase - thanks to YouTube.

A video posted on the site last year showed some inebriated revellers re-enacting the advertisement on what appears to be a Nitelink (with one wit ad-libbing "I don't have a bank account"). Not only has the clip now been viewed by more than 100,000 people, it has also sparked a domino effect of copycat videos and there have been regular sightings of the scene being re-created on buses and the Luas.

Jane McDaid, managing director of ThinkHouse, a PR agency which specialises in youth marketing, says that whether the YouTube post was "genius on the part of the marketers, or simply a couple of kids having a laugh on a bus", it succeeded in making the financial regulator's message stand out from the crowd, without damaging the brand.

Whether it was a deliberate ploy or not, the video clip worked on the principles of guerrilla marketing, which McDaid sums up as "surprising the consumer in an environment or in a way that's unexpected, entertaining and memorable".

Guerrilla marketing tends to be used by challenger brands that may have a tight budget and relies on unconventional and cost-effective branding tactics to outsmart existing heavyweight players. Ryanair, for example, tends to take an aggressive marketing stance, favouring guerrilla-style shock tactics and pushing the boundaries much farther than its rivals Aer Lingus and BA.

Last year, ThinkHouse drove a campaign which saw mobile network 3 effectively ambush the Ireland vs England Six Nations rugby match sponsored by 02. Guerrilla marketing tactics were used to steal the limelight, such as branding the streets with "clean graffiti", (eg, cleaning pavements with high pressure hoses through logo stencils), conducting a laser light show and flying a branded plane over Croke Park. "All the airwaves the following day were talking about how 3's presence had outshone the actual sponsors of the event," says McDaid.

The development of new media, such as the internet, blogs, virtual worlds and podcasting is a guerrilla marketeer's dream, as it opens up new channels of engaging and interacting with consumers. Last year, a "really, really clever" climate change campaign was staged on Second Life using guerrilla tactics, she says. To highlight the potentially devastating effects of climate change, a vast area of the virtual world was flooded, and the campaign reached an audience of millions.

When it comes to reaching the youth audience in a media marketplace that is extremely cluttered, "guerrilla marketing can serve to surprise audiences in places and spaces that would otherwise tend not to have your brand", she continues. But it isn't just a tool to entertain young consumers: it can be used to create "brand fans" right across the spectrum.

Although guerrilla marketing is relatively new to the Irish marketplace, companies are warming to its principles. Powerade's "Never Give Up" campaign featured an ordinary "Joe Bloggs", who took on the challenge of learning to swim with the goal of competing in a triathlon.

The campaign was "really refreshing", McDaid says, as it mixed "above the line" advertising - eg, traditional television advertisements - with new media, while working in an element of reality television.

Although celebrity endorsements for products are still valuable, companies are beginning to recognise that consumers can represent brands just as well, if not better. Consumers are self-publicising, through MySpace, Facebook and their own blogs and have more of a voice than ever, which means that they can make great brand ambassadors.

Guerrilla marketing can tread a very fine line between complying with the law and being slapped with a fine, and at times campaigns go too far. "Turner Broadcasting ran a stunt which cost reputation loss and $2 million in penalties," McDaid says.

The company stuck illuminated boards on the side of bridges and buildings promoting a TV show, which were mistaken for bombs. This led to road closures, traffic jams and the deployment of public safety personnel.

It can, however, be done responsibly. "People just assume with guerrilla marketing that you're out there doing all sorts of illegal things, repainting the Spire or whatever. That's not what properly executed guerrilla marketing is about," she observes.