From Wikipedia:
The Information Research Department (IRD) was a propaganda unit set up by the Labour government in 1948 at the Foreign Office, after the start of the Cold War.
Celia Kirwan, a close friend of Orwell who had just started working as Robert Conquest's assistant at the IRD, visited Orwell at a sanatorium where he was being treated for tuberculosis in March 1949.[2] Orwell wrote a list of names of persons he considered sympathetic to Stalinism and therefore unsuitable as writers for the Department, and enclosed it in a letter to Kirwan.[1] The list became public in 2003.[3]
How damaging was this list to the careers and influence of the people it named? Was it seen only by Kirwin, the IRD, or was it shared to the British Intelligence Apparatus as a whole (or even beyond)?
"A close friend" doesn't quite capture it. Celia Kirwan was the sister-in-law of Arthur Koestler--Orwell's close friend--and he had once proposed marriage to her. That she was put up to the job of getting this list while Orwell was dying was a bit of a hit job.
As to the list itself, I would point you to Christopher Hitchens' analysis of the entire affair. But in short, he didn't "reveal" any opinions that he hadn't already expressed in public, the careers of the people he named were not black listed, and many of the people listed were, in fact, communists.
Celia Kirwan describes how during her hospital visit he gave her the names of friends, such as Franz Borkenau, who could be counted upon—“the worthwhile writers on the anti-Communist front.” Later he sent her a Sellout list he had compiled with Richard Rees and Arthur Koestler. “Anyway, he gave me a few names, and I gave them to my department. And it was as simple as that.”
She does not quite say if he understood what “department” she was working for, but she speaks the truth when she reminds us that in giving her the list “he was only giving me the names of various people who were already very well known to anybody who studied Communism. It wasn’t as if he was revealing the names of spies.”
That last observation is worth underlining. Nothing that Orwell discussed with his old flame was ever used for a show trial or a bullying “hearing” or a blacklist or a witch-hunt. He wasn’t interested in unearthing heresy or in getting people fired or in putting them under the discipline of a loyalty oath. He just wanted to keep a clear accounting in the battle of ideas.