An Garda Síochána can request the information from the mobile phone networks under specific circumstances. When you turn your mobile phone on, it automatically connects your phone and Sim card to the nearest mobile phone mast, giving you access to the network. Likewise, the phone will regularly connect to base stations around it as you go about your day. If you are travelling, your phone will connect to different base stations along the route as you move. Once you have a second mast to ping, you can get a reasonably accurate location for the phone.

Source: The Irish Times May 21, 2018 20:37 UTC