Modern smartphones have made it easy to track people, with everything from your Facebook account to your web browser using location-based data.

But the ability to track mobile phones predates the current crop of smartphones, thanks to triangulation.

Where does the information used in triangulation come from?

An Garda Síochána can request the information from the mobile phone networks under specific circumstances.

How does it work?

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. When it does that, it sends two numbers, one identifying the Sim card and the other identifying the phone (the IMEI number), to the network.

If you make a call, that information is also sent 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. Information from the base stations can then be used to triangulate a location.

How much information is sent?

In the past, phones would connect to networks every time there was an event – calls, texts, and the odd check-in with a base station. That all could add up to a regular stream of location updates. But modern smartphones are constantly connected, sending emails and working in the background to update your Facebook feed and Twitter timeline. That means there is a huge volume of activity, with your phone updating its location every time it connects to the network.

Is there any way to stop that?

Yes: turn off your phone. You can switch off location services and background data, or even turn off data altogether, but as long as your phone is turned on, it can contact a base station.

How reliable is triangulation?

The triangulation is pretty reliable, experts say. There are many more masts in a city than there are in rural areas of Ireland, but each time your phone connects to a mast, it will also give away how far away from the mast its current location is. Once you have a second mast to ping, you can get a reasonably accurate location for the phone.

How has it been used in the past?

Triangulation data has been used in several high-profile cases, including the trial of Graham Dwyer for the murder of Elaine O’Hara and the murder trial of Joe O’Reilly, convicted of killing his wife Rachel. It was also cited in a civil case relating to the Omagh bombing.