Like swallows heralding autumn, hordes of screeching Spanish teenagers let us know, in a way the weather usually fails to, that summer is here. They might block the pavements, deafen you on the bus and take up seats in your local, but they bring in lots of hard cash and add a bit of colour to our streets. How would you feel about one in your home? A guest at every meal, a silent witness to your family rows, astonished eyes monitoring your eccentricities, strangers' hairs in your bath, not to mention all that extra cooking and washing up. And having to talk to people with whom you have nothing in common as you stagger around first thing in the morning or arrive home brain-dead after a hard day at work. Would you put up with that for under £100 a week?

"I love it," says Fidelma McMahon, who has been welcoming foreign students into her Killiney home since 1985. With four daughters of her own (ranging in age from eight to 14) and a foster daughter aged four, Fidelma has no problem acting in loco parentis to the stream of students from all over the world: "We treat them like family. We all sit around the dinner table together and on their first day, they have to sing a song from their country and we sing an Irish song. "We have a lot of music in the house and the students get involved. We've had a Japanese student who went home playing the tin whistle, a Mexican who learnt the violin and an Italian who loved Irish dancing. The last Japanese girl we had to stay was here for almost a year and she really became one of the family - she would even cook the dinner when I didn't feel like it. It was a shock that she was going home."

However, Fidelma stresses the importance of rules. "On the first day, I take them around the area to show them the places they shouldn't go to. One boy ignored me and ended up in hospital covered in cuts and bruises, so I took him to the guards to teach him a lesson. And I don't say `help yourself to food' because you don't say that to your own children."

"There are two potential problems: the family could be dodgy, or the host family could be stuck with a monster," says the director of one Dublin language school. Most schools visit families "to check the student has a clean bright room and the family are nice, friendly people who are prepared to take care of children", explains Anne McCrudden of the Dalmac Language Institute in Rush. With the present-day fear of child abuse, schools may also check out a general area with the guards, to see if there are any houses where children should not be allocated. According to Rosemary Quinn, director of the Centre for English Studies in Dublin, "Ninety per cent of Irish families are good at hosting. They really take it seriously. On one of our junior programmes, a boy broke his leg two weeks into his eight-week visit and the family visited him regularly in hospital; his own parents remarked on the high level of care from the Irish family."

The most common complaint from students is the food - "too many potatoes and overcooked pasta" - but other mysteries include carpet in the bathroom (the Japanese), separate taps ("How do you wash your hands?" wonder the Spanish) and thanking the bus driver (the Italians). Cultural differences are a source of great amusement, but Fidelma believes that her daughters have benefitted from being exposed to other cultures - they can all speak a little Japanese - as well as from learning to share their things, especially the bathroom. Not all students will fit into your family, warns Rosemary Quinn, because families and agencies abroad fail to provide schools with sufficient background information to match the student with the host family. Occasionally, the problem may be more serious: "It is not fair to send children away with behavioural problems as this often makes the problem worse and they start stealing or something," says Rosemary. Stealing, though, is only one problem. Fidelma had a Spanish teenage girl who arrived with "a wardrobe full of pills" and another who refused to take a shower or change her clothes. One worried family requested that a student be sent home early after she took to walking around in the rain barefoot until her feet were bleeding.

The director of a small Dublin language school was forced to expel a 16-year-old Spanish boy who stole from other students and set off the fire extinguishers. It was worse for the host family and their neighbours, he says. "He killed cats in the area and brought home the ears he had cut off. The host mother noticed her eyes were stinging and before he was deported he told her he had been urinating in her contact lens case." For each horror story, there are thousands of families who build lasting friendships with their foreign visitors. "I take up to five students," says Cassandra Ferguson-Denny in Kimmage, who began hosting because she loves to cook. "Lots of my students come back for years and sometimes their parents visit me. With one Italian family, the son and the daughter came to stay and then the mother. Then I stayed with them for two weeks in Rome."

That is nothing compared to the Irish family which was given a house by a foreign guest. As a teenager he spent five summers here and continued to visit whenever he was in Ireland. The two families became increasingly close, as they attended family weddings and exchanged Christmas presents. When the Irish family encountered financial difficulties in their business, the student's family paid off the mortgage so they would not lose their home. Another lucky family was lent a holiday home in Brittany by one of their regular foreign visitors.

Many foreign students find romance in Ireland and some even find it in their host family. "When one Italian girl arrived at her host family in Monkstown, the son of the house opened the door and it was love at first sight," remembers one English-language teacher. "There was a big romance, she came back to visit a few times and they are married now."

For most families, the option of hosting is becoming less attractive. More women are working outside the home and have no desire to come home to making up packed lunches for assorted foreigners. Jackie Joyce, of Marketing English in Ireland, says that schools are struggling to find host families because the numbers of students are up 20 per cent and women who used to take students have now reared their families and are going back into the workforce. According to Hilary McElwain of CELT language school, the situation is approaching crisis: "I think many schools will have to turn down bookings because there is a shortage of host families. My educated guess is that student numbers are up 10-15 per cent but that host families are down 25 per cent."

The Government is also blamed. "The Celtic Tiger has a down side," says Jackie. "People are more prosperous and in addition, they don't need the income since the Government abolished college fees. A lot of people would take students to save up a lump sum for the fees in September. We noticed an immediate slump after the abolition."