As part of their three-month undercover investigation, Spotlight journalists followed the trail from Lithuania through immigration at Dublin to the farmlands of Northern Ireland.
The immigrant workers are paying up to £1,000 to be trafficked to the province.
Employment agents in Northern Ireland with contacts in agricultural sector are passing them on to farmers who want cheap labour.
The illegal workers are promised good salaries and good conditions, but the reality turns out to be very different.
On one farm, Lithuanian journalists posing for Spotlight as immigrants were paid about £1.20 an hour - a quarter of the UK minimum wage.
Elsewhere, Spotlight found illegal workers living in an unheated barn with no toilets or washing facilities.
One of the Lithuanian journalists, Saulius Jarmalis, said: "I am amazed by the things I have found here.
"I will tell all my friends now to warn people not to go to Ireland to work. Don't try it and don't believe the promises they give you."
The programme also discovered a hostel in County Armagh operated by a former chairman of the Northern Ireland Labour Party, and now prominent member of the Northern Ireland Housing Association, Erskine Holmes.
Until recently it was promoted as a youth hostel by the Northern Ireland Tourist Board. Now, it houses 16 alleged illegal workers.
The investigation found the workers pay Erskine Holmes £15 a week each for their beds.
Mr Holmes runs the hostel in the name of a charitable company, The Blackwater Valley Museum.
Using a secret camera, Spotlight journalist Declan Lawn posed as an agent for illegal immigrants and paid Mr Holmes £150 to house some invented workers.
He later confronted him to ask for his comment, but Mr Holmes drove away without answering any questions.
As well as the profits made in Northern Ireland, eastern European agents are making hundreds of thousands of pounds from trafficking workers to Northern Ireland.
The programme concluded that the authorities in Northern Ireland seem unable to do anything to stop the flow.
Spotlight is broadcast on BBC One Northern Ireland television at 2235 GMT on Tuesday.