Sage Group, a provider of accounting, payroll and payments software for businesses, said an internal login had been used to gain unauthorised access to the data of some of its British customers.
The personal details of the employees of about 280 British companies were potentially exposed in the breach, a company source said. It was working to ascertain whether any data had been stolen, the source added.
"We are investigating unauthorised access to customer information using an internal login," the company said in a statement.
"We cannot comment further whilst we work with the authorities to investigate but our customers remain our first priority and we are speaking directly with those affected," it added.
Sage, one of Britain's largest technology companies, says it has more than 6m small and medium-sized businesses using its software worldwide, including in Ireland.
It said last month it was confident its revenue would increase by at least 6% in the current year ending next month, continuing a pace set in the six months to end-March, when revenue rose 6.2% to £747m.
In the same period its operating profit rose by 1.9% on a like-for-like basis to £189m.