Nearly a quarter of young workers admit to accessing the dark web in the workplace.
A new survey among 500 Irish office workers also found that one-in-five people spend over an hour of company time on personal matters every day.
Meanwhile, 42% said their boss had caught them in personal activities like reading the news, checking social media and messaging friends and family - and had no problem with it.
The survey was carried out by IT services provider DataSolutions.
The company's Group Security Director David Keating said companies should act to ensure their servers are not put at risk.
"If employees are engaging in personal activities at work, and perhaps interacting with unsecure websites or entering confidential information, company servers could be more susceptible to cyber-attacks," he said.
"It’s not about banning personal activities altogether; it’s about being aware of potential weak spots and introducing technologies that help to protect work systems and data.
“It’s not only information and infrastructure that’s potentially at risk here, it’s the company’s reputation and customer base.
He said Irish businesses should ensure that, "regardless of what staff are doing, they are taking action to shield their organisation from threats."
The survey found that people aged between 16 and 23-years-old were most likely to carry out personal tasks at work - and were also most likely to access the dark web.
The older the employee, the less likely they were to do so.