When Joseph and Isaac, two west African fishermen, flew to Belfast earlier this year, leaving behind the coast from which slaves were traded for centuries, they had high hopes of a better future. A recruiting agent in their home country had secured them jobs on UK-flagged trawlers and they had transit visas and letters of guarantee for British immigration from the boat owners.

Their 12-month contracts promised them £1,000 a month. They did not specify hours or conditions and allowed the owner to withhold the first month’s wages but it nevertheless seemed undreamed of wealth, which would be enough to pay off the £1,000 agent’s fee which they had borrowed and then start saving for their families.

They were waved through immigration at Belfast airport thanks to the letter from the owners saying that they would be transiting straight away to international waters from Belfast harbour. No one was at the airport to meet them and they had no money or phone but airport staff made a call for them to the boat owner and told him to come and get his lads.

But when he arrived they were driven not to Belfast harbour but across the Irish border to another port in the Irish Republic. The men had no idea they were in a different country nor that they had become illegal, like many others brought in by the same route as cheap labour for the Irish and UK fishing industry.

Transit visas for non-EEA migrants are not intended for fishermen working in and out of European ports, only for seamen passing through to cargo ships that spend months at sea. They had no permits to work on Irish boats and no permission to be in the UK.

They were told that night that they would be living on the boats. After a while, they were driven in a windowless van to another Irish port to join other trawlers. They said they had no idea where they were, and that they were worried but felt they had no choice because they had incurred debts to come and had to pay them off. Joseph said it was hard to sleep because the berths were too short for them to stretch out fully, and he quickly developed back pain. They worked onshore mending nets and on fishing trips for a month before being paid.

Joseph and Isaac had been fishermen back in Africa but were stunned by the difference in conditions. Back home boats tended to go out for six hours with more crew and then return to land their catch. Fishing out of Ireland involved a week or 10 days at sea with almost no rest. Once the nets had been shot and the first trawl pulled in, the hours were unlimited. “You don’t count hours, it’s all the time, all the time,” Isaac said.

They complained that the Irish crew slept when nets had been shot while “the black men are given the heaviest jobs”, according to Joseph.

As well as handling the nets, they were required to process the fish onboard. What used to be factory jobs on land have been moved on to trawlers to get maximum economic benefit from labour. For much of the time they were fishing for prawns.

“It’s the processing work that kills you,” Isaac said. When the nets were hauled, they would tip the prawns on to a hopper, and apply a chemical powder to kill them, which made the workers’ throats burn. Then they would work tailing the small prawns, and arranging the bigger prawns in boxes to carry into a blast freezer.

Next they would have to take out the frozen boxes, label them, and move them down to the cold store. Their hands would freeze with all the cold water and were so numb they couldn’t take their gloves off, so they ate with them still on. When the Navy came to inspect, they said the skipper would tell the crew to hide the big fish which they gutted and processed too but weren’t supposed to catch. As soon as they had processed one batch, it would be time to haul the next catch in. They said they worked 20 hours in every 24 for seven to 10 days at sea at time.

“You couldn’t stop, you would be kicked off the boats. They would shout at you: ‘You Africans are bad, you fucking black bastards’, like that. I was angry inside but I knew I couldn’t let it out because the boss would use my month’s pay to fly me home,” Joseph said.

They were transported around Irish ports and became aware of dozens of other undocumented migrants in different ports. They said they had to ask permission to go on land and were told at one point not to leave the vessel.

Eventually they asked for help. They have now been given leave to remain in Ireland as part of the state’s arrangements for tackling modern slavery.

“It was a shock to be talked about as modern slaves; we thought slavery had gone away. We’ve been trapped. My mother was so upset when she heard, she ended up in hospital. She’s ok now, she knows we are ok, but I feel very bad,” Joseph said.

• The fishermen’s names have been changed to protect their identities