The checks were triggered after controlled drugs were found to be missing from the ship's medical station.
Stena Line said it has a zero alcohol and drug policy for staff, unless medication is prescribed by a medical professional and does not inhibit the performance and ability of staff to carry out their work in a safe way.
The company has not confirmed how many staff were tested and how many employees failed the tests in the incident last month.
A Stena Line spokeswoman said: “Stena Line can confirm that as a result of random testing, a number of employees onboard its Belfast–Cairnryan operation have failed the test and as a result Stena Line is now carrying out a full investigation as part of its established disciplinary process.
“We can confirm that it was identified through routine inspections onboard that controlled drugs were missing from the ships medical station that is in a secure location onboard the vessel. For this reason we carried out a drug and alcohol testing.
“The police were notified immediately. The Company will not comment publicly in detail on matters of employment.”
Belfast Harbour Police confirmed it had been called in and that the matter remains under investigation.
Belfast Telegraph Digital