By 5.45, when the 22nd finisher in the Anglo Celtic Plate 100 kilometre race had crossed the line in the Phoenix Park, the heavens had opened and a nasty swirling wind had made running conditions anything but pleasant.

The weather had driven the Saturday afternoon joggers and strollers home and most of the race finishers were long since tucked up in their north Dublin hotel. But Pauline Walker of Scotland was still running. At eight o'clock that morning she was one of 27 competitors to start the race (four failed to finish); by 5.45 she still had 12 miles to cover, on her own.

Those of us unfamiliar with the stuff of which ultra-runners are made didn't rate her chances of finishing too highly. Each time she completed the 1.6-mile circuit, a lonely figure painfully inching past us as we sheltered from the rain in the tent at the finishing post, you doubted that she could make it to the end. At least not before the 12-hour limit expired.

You wanted to tell her to pack it in, throw a blanket over her shoulders and bring her into the tent, out of the rain, but then it dawns on you that the maxim that guides the life of an ultra-runner is "I've started so I'll finish". You could see it written on Pauline Walker's face. Of course, she finished. At 22 minutes past seven, exhilarated, she crossed the line. "Unbelievable," said Mick Kennedy, co-organiser with Maurice Mullins of the biggest ultra-race event yet to be staged in Ireland. "I run ordinary marathons and I think I'm a hero, but these people? They're something else," he said.

"I know, I know, you all think we're mad but. . . but. . . you're probably right," laughed Stephen Moore of England, who won the race in a mind-boggling six hours, 56 minutes and looked like he could probably have run for another seven hours when he chatted after his triumph.

So, what is ultra-running? Well, the definition of an "ultra-marathon" is "any organised footrace extending beyond the standard marathon running distance of 26 miles, 385 yards". Ultra races typically begin at 50 kilometres and can extend to frightening distances - there is no limit to the challenges these runners take on.

The races can be point-to-point or run on loop courses, like the Anglo Celtic Plate which, until Saturday, had only ever been staged in England, Scotland and Wales.

There are two types of events - those in which runners set out to cover a fixed distance (whether it be 100 or 1,000 kilometres) and those in which runners attempt to cover the greatest possible distance within a fixed period of time (such as 24 hours, 48 hours or six days).

One of the most famous ultraraces is the Spartathlon, a 250 kilometre marathon between Athens and Sparta, completed last year by Costas Reppos of Greece in 25 hours, 11 minutes. But tap ultra-racing into your Internet search engine and you'll discover that an endless number of equally daunting races are taking place annually in every corner of the planet. "It's not that we're mad, it's just a challenge for us, we want to see what we can do," says 51-year-old Eleanor Robinson, the second woman home in the Phoenix Park on Saturday (after English team-mate Sharon Gayter, who finished in eight hours, 27 minutes). "You don't always have a good day, but you can always keep going. Even if the time is slow, you have the satisfaction of knowing that you've conquered it, you haven't let it beat you.

"That's why we persevere, even though you'll see some runners in the most dreadful state. You think: `Why are they doing it?' but they've got an inner drive that says `I'm not going to let this beat me'. That's what ultra-running is about. Yes, the races are competitive, but the real competition is within yourself."

Robinson has been an athlete since she was 14-years-old but soon discovered that endurance, rather than speed, was her strength. "The only problem was that when I was a young girl, women weren't allowed run long distances but then, in the early 1980s, when the marathon was open to women for the first time, I ran one and loved it. Soon after I saw a 12-hour race advertised and I thought: `I'll have a go at that'. I broke three world records I didn't even know existed. I'd found my home in athletics."

Last year Robinson marked her 50th birthday by running 1,000 miles across Australia. "Well, I've been a runner all my life and that's the way I wanted to celebrate it," she said, when I suggested a nice meal and a good bottle of wine might have been an easier way of marking her half century on this planet. She broke the world record by 33 hours, completing the challenge in 13 days, one hour, 54 minutes and two seconds.

"The coolest day was 32 degrees, it was murder, but I figured I'd travelled all that way, I might as well finish the race. I took a three-hour break in the afternoon and two half-hour breaks at night to sleep," she said. "But I stole 10 minutes here and there to nap," she whispered, as if she had confessed something truly awful. Maurice Mullins is Mr Ultra-Running in Ireland. "I'm God's authority on it here, but that's because nobody else knows a thing about it," he laughs. And then he tells you the remarkable story of Ireland's first ultra-runner, Dan O'Leary, from Rathbarry, Co Cork.

O'Leary emigrated to America in 1860 and earned his first fortune (later wiped out in the Great Fire of Chicago) selling bibles and reference books wholesale. Every day he would walk 15 miles out to the suburbs of Chicago to collect payments from customers, and then walk back again. He discovered he had a talent for covering long distances at speed and a few years later, he became only the second man to cover 500 miles in six days.

He returned to race in Dublin and London, becoming one of our earliest athletics world champions, and two years ago, Mullins's campaign to see that his achievements be recognised was rewarded when a statue was erected in O'Leary's honour in his home town. Forty of his great-grand children travelled over from America for the occasion, one of the happiest days in Mullins's life.

Chris Fanning, a native of New Ross, but now living in England, is one of the few Irish athletes, so far, to follow in O'Leary's trail. He was the first member of the Republic of Ireland team to finish on Saturday, in eight hours, 24 minutes.

He's just getting his breath back after completing the 100 kilometres when I tell him about a truly horrendous Canadian ultra-race that I've read about (where you run up and down mountains for some ungodly period of time). Alarmingly, he looks interested. "It's called the `Knee-Knackering Marathon," I tell him, convinced it will put him off. His eyes light up. "Ah Chris, no," I say, but I know it's too late. Ultra-runners? A breed unto themselves.