For some it is a furtive trade that happens in dark corners of railway stations, roadside cafe car-parks and ports. Money may change hands, well-used notes, usually deutschmarks.
In return the buyers get some instructions, sometimes a few phone numbers and forged documents, or stolen passports if they are lucky. Then someone will slide a knife through the canvas siding of the chosen 40ft container. The people will climb inside and wait for the doors to be thrown open in whatever country they want to reach.
For others it is more haphazard. They meet in groups in European countries where they have applied for asylum or are simply illegal, and talk about other routes to better places. It is a system of "helping hands", according to one source at the French port of Cherbourg.
In France they call them les clandestins, the young men, and occasionally women, who smuggle themselves or are smuggled on to containers, and wait for days or weeks in searing heat or cold with little water or food.
The Romanian authorities believe that Russians, Poles and Hungarians are involved in the trafficking of people, charging for passage on a cargo lorry, bribing port officials and drivers to turn a blind eye, and organising false papers.
This week the deputy head of the EU police agency Europol, Dr Willy Bruggeman, said that at least two Romanian gangs operating in Germany and the Netherlands are involved in the trafficking of illegal migrants. Dr Bruggeman, speaking in Amsterdam, said he also believes the Russian mafia has moved into the trade.
But police at the port of Cherbourg deny that these criminal organisations are based in France.
"There is no Mr Big who smokes cigars and drives a big car," the head of the Cherbourg frontier police, Mr Yves-Marie Robine, maintains. "If there was it would make our job easier, because we could simply arrest him and stop the trafficking."
Asked to comment on the Europol view that Romanian gangs and the Russian mafia are involved, Mr Robine says he believes the trade is organised on the Italian and German borders with France and the routes through France are arranged by these groups.
The frontier police are referred to by some who work in the port of Cherbourg as the "secret police". Their office is in an inconspicuous granite building on a quiet street near the port. The 13 plainclothes officers, a branch of the national police force, have the usual police powers of arrest. They patrol the streets of Cherbourg in unmarked cars and can arrest anyone who does not produce the appropriate identity papers.
There is co-ordination with the national railway police, Mr Robine says. When suspects get on the train to Cherbourg at Paris the frontier police are usually waiting for them in the depressing plate-glass station building.
"Historically, immigration to the west always existed. After the fall of the Berlin Wall people came here to see a different regime."
Mr Robine's force has arrested more than 300 Romanians in Cherbourg since the beginning of the year, more than the total number arrested in 1997.
Romanians applying for political asylum in France must apply to the Office Francais de Protection des Refugies et Apatrides (l'Ofpra) in Paris. As in Ireland, such an application brings with it the entitlement to remain in the country while it is processed. However, in France this procedure is much faster, usually taking up to three months.
One local source in Cherbourg says the French believe that people use their L'Ofpra application as a breathing space to organise the next leg of the journey to Britain or Ireland.
Those arrested in Cherbourg are depicted in coloured bar charts, used to co-ordinate police intelligence. There are Belarussians, Ukrainians, Bulgarians and Hungarians, each represented by a small block of colour. A yellow bar that shoots more than ten times as far across the page represents the Romanians.
One in four of the Romanian people arrested are in France illegally, without the papers needed to stay. These people are tried before a local tribunal which usually rules that they should be deported by plane to Bucharest.
According to local newspaper reports, those who are deported from Ireland also face the tribunal system. In February, a 29-year-old man was sentenced to a month's detention for being an illegal immigrant in France, trying to smuggle himself to Britain. The tribunal also ruled that he should be deported and forbidden to return to French soil for three years.
At Cherbourg the containers sit in an open compound into which anyone can wander. Two wire fences - one shoulder height, the other around 7 ft high - separate people walking from the railway station from the compound of unguarded containers. Those who want to get into the compound do not even have to scale the wire, as a pedestrian gate leads into it across the railway lines.
The hauliers are familiar with the problem. "The port is wide open here. If you have a curtain-sided trailer then it is likely to be slashed at some point," one Irish haulier says. "Other refugees have been known to lie on the top of spare wheels and under the axle."
The French police are more interested in checking their tachographs, which measure how much time the hauliers spend driving, they say, than in checking for stowaways. Containers can remain in the compound for 12 hours or more as they wait for a ferry departure. In midsummer, the heat inside a container can reach up to 40C. Three weeks ago, one driver saw 10 people being taken out of a container that had been on the tarmac for two days. The fear that the containers will become coffins for their human cargo increases as the summer approaches.
"It is not a French problem," Mr Robine declares. "It is a European problem that we are coping with. They come here with the aim of going to Ireland or Britain. They arrive in Cherbourg. If they can't get out they spend one or two days in the town. They meet and swap information. That is the organisation."
The police have responded by treating it as a security problem. Security at the Cherbourg compound is to be tightened later this year. The port town's Chamber of Commerce, in co-operation with the port authorities, is to install closed-circuit TV and pay for a private security night watchman. Already the reports of the private security firm paid for by the Chamber are copied to Mr Robine's office.
The immigration police are to use new detectors which can be inserted into cargo lorries to determine how much carbon dioxide is in the lorry. If there is CO 2 in the container, then breathing people are creating it.
According to SOS Racisme, one of the largest anti-racist groups in France, the Romanian population has not built up a strong community. There are "a lot of material difficulties" in Romania, a spokesman for the group says. And the west is seen as a place where unknown luxuries like chocolate and coffee, as well as the possibility of better-paid work, are available.
One organisation, whose patron is Ms Danielle Mitterrand, the wife of the late president, houses Romanian refugees in the Paris suburb of Creteil. But to most Parisians they are the women in headscarves begging in the doorways of expensive shops.
The Department of Justice in Dublin does not break down the figures for asylum-seekers by country of origin. However, it is understood that the number of Romanians registered has risen from approximately 40 in 1994 to 800 last year.