I'm watching Lord of the Rings: Return of the King and a significant plot point involves a message being sent between two cities by sentries lighting a series of fires across a mountain range, passing along the signal. Obviously, Lord of the Rings is not exactly a realistic movie but I was wondering if there were times and places where signals were transferred over large distances like this? If so I was wondering how reliable it was and who would be at all of the different locations, ready to light the fire signal?
There was a system like the one you mentioned in parts of modern day Slovenia (old duchy of Carniola) and went as far as Treviso and Villach (modern day Italy and Austria respectively). The system was put in place in 1471 to warn the local population of Turkish raids. Apart from bonfires, churches' bells and cannons were also most likely used, especially if the visibility was low (cloudy weather), though primary sources don't mention this.
The system was built to give rural population time to escape with their lives and valuables to fortified churches or tabors, that were located in the vicinity of the bonfire (either same or nearby peak). When the place of the bonfire and tabor were a separate location, hills with the bonfire were called Grmada (a pile), Kres (bonfire), Straža (sentry point) or Peč (oven) when not after the patron of the local church. The most famous case is just outside the capital of Ljubljana, with Grmada peak serving as the bonfire peak and Šmarna Gora peak as the tabor. The tabor on Šmarna Gora is well preserved has a small museum dedicated to the Turkish raids. The Bonfire on Grmada is lit once a year partly in the memory of this historical event, but mostly due to an unrelated pagan ritual.
The people who were tasked with liting the bonfire lived in the vicinity of it. The job to lit the bonfire came down to churches when tabors and bonfires were the same peak, when not, hamlets were build and supplied with up to 100 loads of wood.
The system was said to be inspired by a similar one in Eastern Roman Empire.
For primary sources see "The Glory of the Duchy of Carniola" by Johann Weikhard von Valvasor.
While more can always be said, several possible candidates are mentioned in: Is there any historical basis for setting towers ablaze to signal and communicate over vast distances such as in the third installment of Lord of the Rings when Gondor calls for aid? What may have been Tolkien’s inspiration for this?