5,500 members of a British-based construction union are to join the Technical Engineering and Electrical Union, bringing the TEEU's membership to over 45,000.
90% of the Irish-based members of UCATT, who are mainly carpenters, painters and sheet metal workers, voted to merge with the TEEU through a transfer of engagements.
TEEU General Secretary Eamon Devoy said the move will create a new, enlarged union that would have more power to push for better pay and conditions across the economy, and particularly in construction.
Mr Devoy also said that the merger was an important part of the TEEU strategy to recruit and organise workers in a growing economy.
He said this would ensure that they could not only claw back cuts imposed by employers during the recession, but could allow workers to share in the benefits of the hard won recovery.
UCATT has 120,000 members in Britain and Ireland - and the 1,500 members based in Northern Ireland will remain in the union.