The IRA were determined to gain control of an increase in haulage business between the Republic and Northern Ireland, and to extort protection money, declassified state papers show.
Irish officials were told that an increase in haulage traffic across the border would provide opportunities for republican paramilitaries through the lapse of official customs controls.
The concerns were raised in 1989, some three years before the free movement of people and goods between EU members came into effect.
It came after the Provisional IRA repeatedly bombed the Dublin/Belfast railway line, which caused constant disruption.
In 1989 British embassy officials contacted the Irish government to make them aware that the US state department was issuing travel advice concerning the Dublin/Belfast railway.
Ireland’s ambassador to the US expressed concern about the advice.
In a public notice, the US state department told American tourists that travellers to Northern Ireland should be aware of the frequent bomb threats and disruption to the rail service.
In a briefing about the railway bombing campaign, foreign affairs official Declan O’Donovan said Irish officials had spoken to the office of then SDLP MP Seamus Mallon about the matter.
The “local guess”, officials were told, was that the IRA campaign against the line was connected with road haulage.
Mr O’Donovan added: “There is a brief that the effect of 1992 will be to cut IRA revenue from smuggling, increase haulage traffic, and provide opportunities for the IRA through the lapse of official (customs) controls.
It is not thought that hauliers are paying the IRA to close the line, but rather that the IRA itself, or a section of it, is determined to get control of the increasing North/South haulage business before 1992.
It was understood that the IRA wanted control of some haulage companies, and to extort protection money from others, officials said.
In March 1989, foreign affairs official Seán Ó hUiginn was told by the then joint British secretary that the railway had a greater symbolic value than the electricity interconnector.
In a confidential note, Mr Ó hUiginn added: “The Provisionals undoubtedly would make tremendous propaganda capital out of the authorities’ inability to defend it.”
British officials wanted Dublin to issue a statement drawing attention to the inconvenience to passengers, and highlighting the “topsy turvy logic” of the Provisionals, who protested against border closures and then “disrupted such an important North/South link”.