Tara Mines is returning to full operation this week, following an 18-month period where the majority of its workers were laid off.
The development has been welcomed by the group of unions at Europe's largest zinc mine.
"There is a sense of relief among workers and the communities throughout the north-east with the full reopening of the mine and the first blast to develop a mineral vein, taking place this week," said SIPTU sector organiser, John Regan.
"The last of the miners have just completed a five-day return to work induction training and familiarisation process."
Tara Mines closed temporarily in mid-July 2023, resulting in the laying off of 650 workers.
The owners, Boliden, said it took the decision due to unsustainable losses and cited factors including a decline in the price of zinc, inflation and energy costs.
It said that the temporary closure would safeguard the future of the mine.
In April of this year, an agreement was struck by management and staff at the Workplace Relations Commission to reopen the facility, which included around 160 voluntary redundancies.
Around 50 workers retired or took early retirement while the plant was temporarily shut down and in care and maintenance.
"We must not have a repeat of an extended lay-off period for workers at the mine," Mr Regan said.
"The Government needs to build into all mining licences a requirement to keep them in operation."
"It also needs to have the ability to step in with financial support if required to avoid a mine going into 'care and maintenance’ in the future."
405 workers continue to be employed at the mine.
Many have had to upskill to different roles and responsibilities under the collective agreement struck to enable the plant to reopen.
The plan to reopen the mine has been helped by the recovery of the zinc and lead markets, SIPTU said.
"It is essential that the workers and company deliver a fully functioning operational mine for future generations of workers and businesses in the North East," said Unite Regional Officer, Brian Hewitt.
"We believe that our members have fully played their role in ensuring its operation into the future, the Government and the owners must show a similar level of commitment."
