Sky Ireland has apologised to some 43,000 customers, who were not given a 'cooling-off' period as part of their contract.
A ComReg investigation uncovered the discrepancy, which goes back to 2015.
Sky is obliged to provide customers with a contract confirmation, and a cooling-off period of 14 days. This is generally done by email or by hard copy in the post.
But between September 2015 and September 2018, approximately 79,000 customers were not provided with a confirmation, and 43,000 were not given a cooling-off period.
In a statement, Sky says it "regrets" this failure and "apologises for any inconvenience".
Sky serves 21 million customers across five countries.
Sky Ireland is based in Dublin and provides customers with digital television channels and broadband facilities.