The Irish businesses of Grafton Recruitment have been sold to the Nottingham-based Staffline group.
Grafton, which is based in Belfast, is one of Ireland's largest recruitment firms with turnover of around £75m in 2017.
The deal does not includes Grafton's other businesses in continental Europe.
Staffline has been in the Irish market since 2013 but this deal will significantly increase its scale and presence.
Tina McKenzie, Staffline's Group Managing Director in Ireland, will take responsibility for the Grafton business, in addition to overseeing Staffline's existing businesses across the island.
She said: "Grafton will continue to run independently from Staffline Group companies.
"With this acquisition, we have exceeded our ambition of a £100m turnover within 5 years, and we are dedicated to continued investment in the local market, allowing further growth."