Ring in everything that goes to your tables. That includes cheese, salsa, guacamole, sour cream, salad, tortilla, tea, coffee and everything else that they might consume and enjoy in our lovely restaurant - if they did not expect to pay for it they wouldn't have come out to eat, they would have stayed at home and eaten whatever was in the fridge.

This notice is stuck up over the serving hatch in Dublin's Break for the Border, where all the waiting staff can see it - and be reminded that what customers order and don't pay for, they'll have to pay for themselves. It's 9.30 p.m. on Saturday night, the busiest night of the week. Downstairs, people are paying £5 entrance, which will go up to £8 as the night progresses. There's a nightclub in the basement; a large bar area and dance floor on the ground level; and a smaller bar with seating for 270 diners on the first floor. The whole place holds up to 2,000 people. Tonight, the first floor alone will turn over an estimated £10,000.

The door policy at Break for the Border is over-23s. Fellow restaurateurs in Temple Bar have recently banned stag and hen parties from their premises. However, Karen Burke, the assistant general manager says, "We actively welcome them, because they're good business. And we'll keep on welcoming them." Temple Bar's loss is obviously going to be Break for the Border's very great gain.

There are nine waiting staff on the floor tonight, serving two sittings from an open kitchen: at 7.30 and 9.30. The menu is Mexican and classic American - burgers, fajitas, steaks, potato skins, key lime pie. The cocktails are custom-named for a testosterone-charged night out: Blow Job, Orgasms, Slippery Nipple, Sex on the Beach. Abba's Dancing Queen is throbbing on the stereo.

Ben Johnson (24), Jenny Connolly (23), and Ian Brosnan (22) are all from Dublin and have all been working here for at least two years. They get paid £3.25 an hour, and their shift covers both sittings. On average, they work 25 hours a week. "I love it," Jenny says. "It's night-time work, management are cool, and I'm making more money than a lot of my friends."

Jenny has been allocated a table of 27: a stag party from Newcastle, and she's waiting for them to take their seats. "I have that table because I'm a girl and they'll tip me more," she says confidently. "Yeah, you do get lads saying, `I'm in Room 105 and what are you doing later, but I give as much attitude as I get. And I never clean up after anyone's vomit."

Ben has a table of 20. "It's some girl's birthday. I won't make much from them because they're not eating much and they're drinking off the bill." Drinking off the bill means people are going to the bar and paying cash each time, rather than adding the drinks to the total, which will have service charge added to it. "It's paranoia," he says knowingly. "People thinking that the person next to them is drinking more than them, and knowing that they'll all have to split the bill at the end of the night."

They'll stop serving at 11.30, but it will be at least 3 a.m. before they finish, because on busy nights they then act as late-night bar staff. "No, management don't pay for taxis," Jenny says, amazed at the question. Do they sit and have a drink together when they finish? "Oh yeah. If there's been a big profit that night, we'll get one drink on the house. Otherwise it's just usually staff prices: £2 a pint."

What about tips? The vexed area of tipping is the greyest one within the waiting service. Tips are meant to be declared in tax returns. It's no secret that waiters and waitresses have long considered tips to be a private gift between themselves and their customers. At this restaurant, there is a service charge of 10 per cent for parties of eight or more. "We do get that in our tips," Ian says. "Not all restaurants do." They reckon they'll make about £60 each on tips tonight, which will be shared out equally. The most they remember making in a night was £140.

The seating arrangements in the restaurant of Break for the Border are very simple. Long tables run along the mezzanine which overlooks the ground-floor dance floor and bar. The placemat is a serviette. Settings consist of a knife and fork. There are no glasses on the tables, because people bring their drinks straight from the bar. The chairs are so close together that Jenny's stag party take their seats by getting up on top of them and walking along to the end of the row.

It's 10.30 p.m. and the place is jammed. The waiters are coming and going from the galley, balancing huge circular trays that hold six main courses. They move with the choreographed precision of dancers. It's a tiny area, in which I count 12 people at one point, but there is still a sense of order about the place.

Ian has 30 at his table. "It's a party," he reports, "They're all ordering steaks. That's often a nightmare, especially at a stag, because everyone wants them done different ways and they're usually a bit drunk, so they just grab whatever's on the tray before we have a chance to find out which steak goes with the right person."

Jenny's stag party have had their starters and are now dancing on the table to Don't Stop Till You Get Enough, in anticipation of the main course. The only problem is that the ceiling is very low, so they are obliged to do a stooping, shuffling, Mexican wave sort of dance, as their heads make regular contact with the ceiling.

They're ordering drinks from Jenny. "I've just brought them 17 pints of Guinness. The rest of the orders were lager and gin and tonic." How many glasses does she carry at a time? "Do you know, I haven't a clue," she says, sounding as puzzled as a natural cook being asked for a recipe. "I never think about it."

Ian was doing applied biology in Tallaght RTC. "I wasn't into it," he says. "So I started here, to give myself time to see what it was I wanted to do." "Working here is a stepping stone," agrees Jenny. "Most of us still haven't decided what we want to do. I'd like to own my own restaurant some day. I don't want to be waitressing forever."

It's well past 11 p.m. They all scatter to keep an eye on their tables. I follow them out of the kitchen five minutes later. Jenny, who has been unfussed and smiling all night, is sitting on one of the chairs vacated by the Newcastle stag. She is not smiling now. She looks as if she's going to burst into tears. "They've gone without paying," she says. "They're down on the dance floor", and she points to a few of them in the scrum below us. "Sometimes tables walk out without paying," Ben confirms. "Then you have to pick up the tab yourself." For 27 people? Jenny nods a miserable confirmation. Has it happened before? Yes.

"Well, we get to keep our part of the service charge," Ben explains. "And lots of other restaurants in town wouldn't. So it works out. Well, kind of works out." Ben is on his way back to his table, where the customers are waiting on 23 pence change from their bill. Ian reports that his table's bill was £391, to which they added a £15 tip between 30 people.

An announcement is made over the tannoy for Jenny's stag party to return to the restaurant. Some of the party bolt out the door when they hear it. Others dig into the pockets and pool some of the bill, but they're still short by £127 out of £540. Eventually, two of them come upstairs and clear the rest of it on a credit card, with bad grace. They are unprintably rude to Jenny, who takes their card, swipes it, and gives it back with a tight little smile. There is no question of a tip: at this stage, she is lucky not to be paying for their entire meal.

"Actually," Ian confides, "we're planning to go to Australia. Next February. Flying to Sydney. Five of us from here, including Jenny and Ben. We want to work in a vineyard and we can't wait. As far away from Ireland as possible," and they head off on a break, clutching big mugs of tea.