The campaign was created by Bartle Bogle Hegarty's BBH Labs.
The billboards feature block black text on a red background, that read: "71 DEAD", "AND STILL NO ARRESTS", and "HOW COME?"
Speaking to Vice, one of the campaign’s organisers, Yvette Williams, said the aim of the campaign was to keep the disaster of 14 June 2017, in which at least 71 people died, fresh in the minds of the public.
A public enquiry started in September and is ongoing – but according to Williams, an interim report that was due by Easter is no longer happening.
She said: "We want the truth. We want prosecutions. People up and down the country need to feel safe in their homes. None of that is happening. We think they're playing with time, hoping that the story will be downplayed."
In the film Three Billboards, Mildred Hayes, played by Frances McDormand, rents three billboards near her home, seven months after the rape and murder of her daughter, to proclaim her dissatisfaction with the police investigation. They read: "RAPED WHILE DYING", "AND STILL NO ARRESTS?", and "HOW COME, CHIEF WILLOUGHBY?"
McDormand is hotly tipped to win her second Oscar for her performance in the film, which is also a contender for Best Picture.