NEW YORK — If there’s one thing Gabriel Byrne has learned in recent years, it’s the importance of a comfortable chair.

After a marathon 106 episodes as psychologist Paul Weston on the HBO drama “In Treatment,” Byrne stars in “Vikings,” History Channel’s first full-length scripted series. He plays Earl Haraldson, a Norse chieftain with a flowing salt-and-pepper mane (all his own, thank you very much) and a taste for cruelty.

Despite the considerable differences between the shows — one set almost entirely in a shrink’s office in brownstone Brooklyn, the other in 8th-century Scandinavia — they both left Byrne, well, uncomfortable.

“My biggest problem was always the chair. I would spend more time testing out the chair than anything else,” the actor said of his “In Treatment” experience. In “Vikings,” filmed in his native Ireland last summer, Byrne’s Haraldson can mostly be found perched in a regal throne, ordering his subjects on another plundering mission or condemning them to death via public beheading.

“Thinking that it wasn’t going to be anything like ‘In Treatment,’ I looked at the throne and said, ‘Yeah, it’s a throne,”‘ he said. Alas, he was wrong. The throne was too big, which may account for the sinister snarl that Byrne wears on his face in so many of his scenes.

Make that “wore”: In Sunday’s episode, Earl faced off in a death match with Ragnar Lothbrok, the visionary young upstart played by model-turned-actor Travis Fimmel. It didn’t turn out well for the aging despot, who was dispatched to Valhalla with a blow from Ragnar’s ax and, finally, a merciful slit of the wrist.

While it was something of a shock that “Vikings,” which has garnered an impressive 5 million viewers a week and was recently renewed for a second season, killed off its biggest star so early in the series, for Byrne, 62, the finite commitment was a major selling point.

“I was kinda happy to die,” he said.

Byrne’s willingness to go gently into the good night certainly had something to do with the grueling pace he maintained on “In Treatment.”

That project required Byrne to be on camera for virtually every dialogue-heavy scene over three seasons that ran as long as 43 episodes apiece. Even the world’s plushest armchair is going to start to feel oppressive at that point.

“In terms of pages shot per day and the volume of material, I think it was pretty spectacular,” Byrne said. “I don’t know that anybody else would have done that amount of sheer volume of words.” Although he is proud of “In Treatment,” he was “delighted” when it was canceled in 2011, Byrne said.

The show’s passionate fan base was less excited about the news. Though ratings were modest, Byrne was honored with two Emmy nominations and a Golden Globe award, with his role as the troubled yet compassionate Weston burnishing his reputation as the thinking woman’s sex symbol. Such was the fascination that The New York Times even dedicated an article to viewers’ erotic fixation with his subtle body language, dubbing him TV’s “latest Dr. McDreamy.”

Byrne said he isn’t surprised by the reaction.

“Being listened to and being heard is an experience that doesn’t happen terribly often,” he said. “To listen compassionately or nonjudgmentally to another person — not to get too heavy about it — but I once heard somebody say that was a form of real prayer.”

As for “Vikings,” an incentive to taking the role was the chance to finally collaborate with “Vikings” producer Morgan O’Sullivan, a friend since Byrne’s early days as an Irish soap star.

“He’s not an actor who just turns up; he’s a creator as well,” O’Sullivan said, citing Byrne’s work as a producer on the films “Into the West” and “In the Name of the Father” and his memoir, “Pictures in My Head.” “He’s one of the most erudite actors I know, and he’s an exceptionally good writer in his own right.”

Even a brief conversation with Byrne, who studied archaeology and Gaelic literature at University College Dublin, bears this out: He’s prone to articulate riffs about subjects such as the similarities between Viking raids and drone warfare or the current malaise in his native country.

But according to O’Sullivan, he’s also a guy “that you’ve got to convince,” and a limited time commitment was an essential part of the deal.

“Vikings” was also something of a homecoming for Byrne.

Though he was born and raised in Dublin, where his father was for a time a barrel maker at the Guinness factory, the New York-based actor hadn’t worked in Ireland since filming “Into the West” there in 1992.

In a coincidence of timing, he followed “Vikings” with “Quirke,” a BBC series based on the novels of John Banville that also filmed in Dublin, a city that has transformed dramatically over the past two decades.

“Part of me felt like a ghost wandering through the edges of a world that I used to know. It’s a very strange experience, the return to somewhere that, to a great extent, exists only in your imagination,” he said with the casual lyricism seemingly only an Irishman can muster.

Byrne confessed that he was particularly nervous on his first day filming “Vikings.” The home crowd of several hundred extras could have had something to do with it, but then again, he always feels a little jittery at the start of a new project.

“No actor who’s any good can say truthfully to themselves, ‘Yeah, I’m good, I’ve got this sorted,’ ” he said.