The meeting-table in the London office of Paul Kenny, the general secretary of the GMB union, is home to a stone taken from the ruins of the cottage where Jim Connell was born, in Kilskyre, a village north of Kells, Co Meath.

The white-washed stone is shown to every visitor. During the London dock strike, in 1889, prompted by the sight of a railway worker raising and lowering his signal flag on the platform, Connell wrote The Red Flag, the song that became Labour's anthem,

“I am very proud of that,” says Kenny, who ensured the GMB backed the first commemoration of Connell’s life.

“It tells everything that anyone needs to know about protecting the rights of working people.”

On Sunday, Kenny is bringing 1,000 GMB union delegates to Dublin for the first time since their predecessors gathered in the city in 1891 to celebrate winning an eight- hour day for gasworkers.

For Kenny, challenges facing workers in Britain, but not just there, are growing, worsened, he believes, by the cuts in wages caused by the influx of people from eastern Europe and southern EU countries.

The union leader, who has connections with Ballygar, Co Galway, says he is not targeting migrant workers. "Blame the exploiters, not the exploited. I'm old enough to remember the 'No Irish' signs. Because of my accent I never got that treatment, but I know my dad did and my uncles did and my brothers did. So it is a pretty sore subject," he tells The Irish Times.

David Cameron’s focus on migrants’ welfare claims is nonsense, he says. “Stop benefits? I’ll take you on any train coming into London at six o’clock in the morning. Those trains are packed with people going into town. They are not going into town to claim benefits, they are working. They are working people.

“I could show you hundreds of people waiting for a day’s casual work. The last time I saw that was at Cricklewood Broadway with the Irish lined up waiting to be picked by subbies.”

Kenny says the British prime minister has no prospect of curbing the EU’s free-movement rules in upcoming negotiations on the UK’s membership terms. But he must return with changes to soothe demands from within the Conservatives.

Expense of workers

Those changes, Kenny fears, could see the relaxation of employment law, health-and-safety rules and holiday pay, all at the expense of workers.

Following such an outcome, the GMB – which has nearly 700,000 members – would urge a No vote in the referendum to be held before the end of 2017, he says.

“We have always been at the heart of Europe and invested in that. We hope to influence change. But what happens if somebody changes the rules so dramatically that we are expected to say, ‘Yes, please, sir, can you beat me again?’

“I don’t think the level of discontent should be underestimated if what happens is that Dave Cameron, in order to satisfy some of his backbenchers, comes back with changes that affect workers.”

The UK Independence Party won four million votes in May’s general election: “Why do these people vote Ukip? You can stick a label on them and say they are racist, but that doesn’t work. It may be a cheap shot but it is deeper than that. They have been touched by, or they have seen the exploitation of, migrant labour driving down wages in the UK.”

Kenny has returned from a weekend conference in the Vatican, at the invitation of Pope Francis: "[He's] very much into supporting workers' rights. He's advocated people joining unions. For someone like me, that gets me back to Mass, I can tell you that."

While at the Vatican, Kenny raised the subject of British employment practices, where sharp falls in unemployment often mask increases in part-time labour, forced self-employment and zero-hour contracts.

“How do you build a family life on zero hours? It’s beyond me. There was an Italian, previously a finance minister who was scratching his head. A Dutch guy with factories in the UK was doing the same.”

Erosion of rights

He does not want to see the UK quit the EU, he says, but he warns the erosion of workers’ rights is seen as “the way forward” by too many: “This is just madness, absolute madness.

“I am not sure that they fully understand where they are driving people like us. I would be really quite upset if we get to a position where we are so disillusioned [we urge a No vote].

“The four million Ukip voters will not go away. There will be more voters who will want out for all sorts of ideological reasons that we wouldn’t support,” he says, but he warns that some unions, such as the RMT, have long urged the UK to leave.

He despairs of the stand taken so far by candidates to replace Ed Miliband as Labour leader. "I am amazed that Labour people are saying that they will be supporting staying in when they haven't seen one dot on a page yet. What is that? A blank cheque?"

On Monday, President Michael D Higgins is coming to address the conference: a stark illustration, in Kenny's mind, of the treatment the union receives in the UK from politicians and business.

“That is a great honour. People are so looking forward to that. They will be buzzing about the fact that he is coming. They are very proud of the union – they do think it is a force for good,” he said.

“When you are used to being rubbished, as you are over here, and then you go to the Vatican or to Dublin, where people understand that unions are a force for good. And they do. But here [in the UK] we are demonised. I haven’t got two heads.”