People are being hit with phone bills of up to €700 when they come back from holidays, according to a new survey.
Switcher.ie has found two-thirds of Irish consumers aren't sure about EU roaming rules.
While one-quarter admitted to being landed with an unexpectedly high roaming bill after a trip abroad.
Eoin Clarke, managing director of Switcher.ie, explains why leaving your data switched on in Europe can be expensive.
He says people with unlimited data packages can be caught out.
"If you're calling and texting and you're in the EU, you will pay the same rates.
"So if you have free calls with your provider, you will receive free calls and texts so there will be no extra cost.
"If, for example, you're on an unlimited data plan that unlimited data plan will actually be capped when you're in the EU when you're roaming.
"So if that allowance is exceeded, you're then into additional rates".