The men, all EU nationals, were stopped at Dundalk on Monday morning as they crossed the border in a work van.
They were on their way to pick leeks in the Republic of Ireland's County Louth.
The checkpoint was set up as part of a day-long operation run by the Irish police's National Immigration Bureau.
It saw all south-bound traffic being filtered off the motorway at Dundalk.
The workers, seven Romanians and one Lithuanian, had no identification documents.
They were driven to Dundalk Garda (Police) station where they were held for several hours.
They were eventually allowed to leave when their employer gave guarantees that documents could be produced.
Paddy Finn said his workers had been stopped before, but had never been asked to produce documents and had never been detained.
Mr Finn, who relies on migrant workers, has land on both sides of the border and his workers regularly cross for harvest work.
A Garda spokesman said such operations had been ongoing for several years.
He said that under Irish immigration law, non-Irish nationals are required to carry documentation.
A County Armagh vegetable grower says businesses need more clarity about border controls, after eight of his workers were detained by Irish police on their way to work.