The queries came slowly at first, from tourists taking coach trips from Belfast to the Giant's Causeway. The Unesco world heritage site on the north coast of Northern Ireland was all very well – but could they stop off en route to see the sites where Game of Thrones had been filmed?

The first scheduled day trips linked to the HBO TV series will begin running three days a week from 5 April. "Demand has just been building and building," said Christine McComb, co-owner of McCombs Travel, the Belfast coach company operating the tours. "These fans are like the new-style Trekkies. They are so into Game of Thrones. And the really great thing is, these are not people who are coming to Northern Ireland anyway. They are coming because of Game of Thrones."

As devotees of the Emmy- and Golden-Globe-winning programme await its return for a fourth series, which begins on Monday on Sky Atlantic, fans in Northern Ireland can be forgiven for being more excited than most. The fantasy drama has been a huge global hit, screening to more than 14 million in the US alone. Barack Obama is so addicted to the show that he is sent advance copies of episodes. Yet its main filming locations are not a sprawling Los Angeles studio, but a former shipyard building in Belfast's docks, and on the moors, beaches and forests of Northern Ireland.

In the five years since Game of Thrones began filming in the region, the biggest TV production in Europe has become big news to the Northern Irish economy. The first four seasons brought a direct economic benefit of £82m, according to the local assembly, including wages for cast and crew, hotels, services and tourism, and has created the equivalent of more than 900 full-time and 5,700 part-time jobs in a region of fewer than 2 million people.

Last month the first and deputy first ministers, Peter Robinson and Martin McGuinness, preceded their annual St Patrick's Day trip to Washington with a visit to senior HBO producers in Los Angeles. As Máirtín Ó Muilleoir, lord mayor of Belfast, said: "Anything that feeds the Game of Thrones hype is very good for the city."

Moyra Lock, marketing director of Northern Ireland Screen, the agency that enticed HBO to the city, said: "The economic impact from Game of Thrones is incredible. But it's more than the money. It's also what it has meant in terms of added value." The programme has spawned spin-off industries in extras and post-production, she said, with opportunities for people to acquire skills in a range of key trades from carpentry and special effects to health and safety.

The agency has already built an additional two sound studios in its Titanic Quarter site, and has applied for permission for further expansion. The new facilities and HBO's vote of confidence have kickstarted a boom in filmmaking in the region, with TV shows including Line of Duty, 37 Days and Blandings all filmed in Northern Ireland, along with the forthcoming Hollywood film Dracula Untold.

The Glenarm jewellers the Steensons are among many businesses to be sprinkled by the Game of Thrones gold dust, having been commissioned by the show's producers to make a range of distinctive pieces, from the crown sported by the cruel young king Joffrey Baratheon to the lion pendants worn by his mother, Cersei, and fiancee, Sansa Stark.

"Anything that gets a good bit of close-up on a main character, we have generally made most of those pieces," said Dan Spencer, co-director of the firm with his wife, Brona. "It did surprise us at first that the producers were willing to pay for the highly skilled labour involved." Everyone seemed to know someone who had worked on the programme in one way or another, he added.

So why Belfast? Northern Ireland Screen lobbied heavily to secure the series, acknowledged Lock, and provided the broadcaster with £10.85m in production funding throughout the four series. The Northern Ireland secretary, Theresa Villiers, has cited the government's tax relief on high-end TV drama as another key factor.

But central to the region's appeal, Lock said, are the studio facilities on offer at the Paint Hall, the vast studio space that plays host to the Throne Room set and many other interior scenes, and, in particular, the wide range of geographical locations within a short distance from Belfast. "We sell Northern Ireland as the smallest backlot in the world. Because it's the size of greater Los Angeles, and when you bring someone over from the studios in America, they think, whoa, because they can get anywhere and back in a day," said Lock.

While sites in Dubrovnik, Morocco and Malta have been used as backdrops for the more exotic exterior scenes, a large number of locations in the region double as settings in Westeros, the fictional continent in which the action-fantasy is set. Castle Ward in County Down became Winterfell, home of the House of Stark. A network of caves in Cushendun was transformed into the Stormlands, and a quarry at Magheramorne near Larne, enhanced by CGI, is the setting for Castle Black, the lonely fortress perched on the Wall.

"The golden prize for any production is to be based in a certain radius of the production HQ, and the great thing about filming in Northern Ireland is we do tend to find everything we need within an hour of Belfast," said Robert Boake, the programme's award-winning supervising location manager, who admitted to spending "as much time as I can" trudging through bogs and forests in search of the perfect gnarly tree or open vista for a particularly dramatic scene. "The more I look the more I find, and I know people are gobsmacked at the array of different sceneries that you can find within that short space of time.

"We are at a very interesting point in Belfast, with all the new stages and studios. There is a lovely melting pot in the industry that hopefully bodes really well for the way things are going to develop here."