Johnny Depp has described his close friendship with Gerry Conlon, saying he would have “taken a bullet” for him, in a moving foreword to a new biography of the man who spent 15 years in prison after he was wrongly convicted as an IRA bomber.

The unlikely friendship between the Hollywood actor and Conlon, one of the Guildford Four, began when the pair met in the US in 1990, the year after Conlon had his conviction overturned and was released from prison. Their friendship was sealed at a gig by the Pogues, where Depp described Conlon and his brother as looking “just like the miscreant, unhinged maniacs I always tended to hang out with”. Depp describes the book, In the Name of the Son, as “a story of a man I loved”.

Depp recalls his first lunch with Conlon during which the latter suggested the star play him in a film about to be produced, which became In the Name of the Father and starred Daniel Day-Lewis as the young Belfast man who was fitted up along with three friends – Paul Hill, Paddy Armstrong and Carole Richardson – for the Guildford pub bombings.

“I was already on the deck wiping away tears (as was he), when he gave the first details of his abduction and torture by the British authorities,” Depp says about that meal with Conlon.

Gerry Conlon leaving the Old Bailey with his sisters after the sentences in the Guildford Four case were quashed on 19 October 1989. Photograph: Peter Macdiarmid/Getty Images

Depp even holidayed with Conlon in his native West Belfast in the summer of 1991, before going on a long road trip to Co Kerry in the Irish Republic to see a famous dolphin called Fungie.

“Gerry had no need to convince me, of course I was going to say yes. Who wouldn’t want to go to a place called Dingle to see a dolphin called Fungie?”

In the Name of the Son is written by Conlon’s childhood friend Richard O’Rawe, a former IRA prisoner in the Maze prison who was spokesman for his fellow inmates in the jail during the 1981 hunger strike.

Conlon left O’Rawe all his private papers, correspondence and other documents pertaining to both his time in prison and his life afterwards. He also revealed to O’Rawe his candid, often painful stories of his long battles with drug addiction after being freed from prison.

In the Name of the Son is published by Merrion Press.