From June 15th, mobile phone operators in EU (+Norway/Iceland/Liechtenstein) will no longer be allowed to charge any roaming fees for phone calls or data usage within this territory (meaning: a cell phone from an operator in EU+ being using within EU+).
Phone operators are still allowed to cap roaming data use at 50MB/month, though most are pushing far more generous caps up to 4GB/month.
This means, for those who like to purchase local GSM cards to use in Europe, that they can use a French number in Ireland or an Italian number in Norway as if it were local. Minutes, SMS allowances, all of that must be interchangeably usable in the same conditions as in the operator home countries.
Switzerland is not covered by this agreement, so double-check if you are planning to travel there and use these facilities.