Brendan Ogle, a senior official with the Unite trade union, has accused his employer of discriminating against him due to his cancer diagnosis and treatment.
Mr Ogle has taken a case against Unite at the Workplace Relations Commission (WRC) claiming he was discriminated against in breach of the Employment Equality Act.
The complaints are denied by Unite.
In opening statements at a hearing before the WRC, Mr Ogle's legal team said his role at the union had been decimated after he returned from sick leave following cancer treatment and that he had been left alienated and isolated.
Lawyers for Unite told the hearing that Mr Ogle has not established that he was discriminated against and that his claim is artificial.
They said Unite took all possible steps to accommodate Mr Ogle but that these were rejected and rebuffed at all times.
In evidence during the hearing, Mr Ogle gave details of his cancer diagnosis in the summer of 2021, a sequence of events that he described as 'borderline traumatic.'
He said he and his wife both had Covid in April 2021, adding that they were quite unwell with the illness.
Mr Ogle said that he was left with swollen glands for some time after and that a swelling under his left jaw did not go away.
He was diagnosed with cancer and underwent surgery in July 2021.
He began chemotherapy and radiotherapy in September 2021.
Mr Ogle said the cancer was aggressive and the prognosis was not good but because he was in otherwise good health he was given the maximum levels of chemotherapy and radiotherapy.
He said he lost four and a half stone and could not eat solids for six months.
Mr Ogle told the hearing that the treatment affected his mouth and jaw area and that he has now grown a beard because of what he described as the 'unsightly' consequences.
In November 2021, Mr Ogle said he had a conversation with the then General Secretary of the Unite trade union Len McCluskey about returning to work once he had recovered.
He said at time he felt at his "very weakest" and asked that a regional officer job in Dundalk be kept open for him.
In follow up emails between Unite and Mr Ogle it was outlined that while the Dundalk role was at a lower grade, his salary would be 'red circled' or protected for a period of three years.
In an emailed reply, Mr Ogle said the pay arrangement was unacceptable and that he was not prepared to be demoted while on sick leave.
When Mr Ogle returned to work in 2022, he said he was shocked by the formal nature of his return-to-work interview, with officials under the impression he had accepted the Dundalk job.
He said he was subsequently told that he was not part of a new Unite strategy for Ireland and he claimed that many of his previous roles had been assigned elsewhere.
"I would go into my office but I wasn't getting many emails," Mr Ogle said.
"I would go for coffee, I would go for a walk, buy a paper, hang around, check my phone, check my emails."
"I was devastated and left scratching my head about what was going on," he told the hearing
"I kept telling myself, it'll work out, we look after each other, we're a union," he said.
Earlier Unite's former General Secretary Len McCluskey said that it was the culture in his time in the trade union to "do the right thing for the individual", and that it was "implicit" in his conversation with Mr Ogle that both roles would be open to him on his existing terms.
"As a good employer we had a duty of care to one of our employees who was asking could we keep the position open for him," he said.
Mr Ogle will continue giving evidence tomorrow.
