This seems to be a little known fact, and doesn't really show up in a lot of internet articles or credited in the film itself-it's not even listed on his IMDB profile either. Shearer, apart from multiple mocumentaries with Christopher Guest, and bit parts in films, is
also well known as the voice of several Simpsons characters- Mr. Burns AND Smithers, Kent Brockman, Ned Flanders, Skinner, Reverend Lovejay etc.

But according to Shearer himself on twitter he did do some ADR for the film.

Most people believe it's the following characters:

Khurgee's the guy in the hanger who reports to Vader about the Millenium Falcon, and then repeats Vader's orders about the scanning crew to the Stormtroopers.

There's an interview with the actor who played Khurgee at a convention he doesn't really sound too much like the film version.

Tiree was part of the ill-fated Gold Squadron, and only had two lines in the movie; the second is perhaps the best remembered ("The guns....they've stopped!")

Sinden was a British actor.

ADR for the rebels was pretty much a major thing in most of the OT. Rogue Three in ESB-the guy who rescues Luke & Han and later dies during the battle-was also a British actor. The Cantina scene was also heavily overdubbed by American actors, Dr. Evazan and Wuher have thick British accents in the rough cut of the scene.

Most interestingly, two of the A-wing pilots in ROTJ are actually women.

The first one appears after Wedge orders Red Two and Red Three to pull in.

She's often mistaken for Arvel Crynd (Guy who destroyed the Executor), probably due to the helmet and ship, and the callsign is notably different. (Arvel is green leader-probably a holdover from some early production stuff that had the A-wings green, something probably dumped due to problems with filming?)

There's also another, older female A-wing pilot that is seen in test footage

For a while I thought she was in the film, dubbed over as one of the A-wing pilots who leads the TIEs into another tunnel in the Death Star shaft.

But it appears that was wrong.

Of course there is eventually a female A-wing pilot in TLJ, the ill-fated Tallie.