The pair were on the plane which came down in eastern Ukraine on Thursday evening, and had been been flying out to New Zealand for the Magpies' pre-season tour.
Mr Alder was a familiar sight at all games, while Mr Sweeney, 28, was also known as an ardent fan who would help organise coaches to away days.
One message left at the feet of Sir Bobby read: "RIP to two of the loyal-ist Toon fans around. No team should lose their life supporting the team they lose. United we stand."
The club laid two bouquets of flowers and wrote: "With deepest sympathy from everyone at Newcastle United."
Another message said: "Always in our hearts. We'll never forget you."
Speaking after laying his tribute, David Mitchinson, 47, said Mr Alder, who was in his sixties, had followed the club since he was a teenager.
DEVOTED FAN: John Alder
“I have known John for 15 years," he said. "We worked together at BT, but he had retired a number of years ago.
“He was just a great guy who had a passion for North-East football.
“He followed the Dunston UTS in the Northern League as well as Newcastle United - but when the fixtures clashed Newcastle United always came first.
“You could always recognise him in a crowd because of his attire. He was probably the club’s number one fan. "
Thomas Concannon, 22, said fans are already discussing permanent ways to mark the deaths, such as creating a flag to show at every game in their memory.
“It is horrible," he said. "John was the ultimate Newcastle United fan and was a legend with his black suit.
"It was only the other day we were going to Oldham and we were talking about him and wondering if he was going to New Zealand – obviously should have known he would.
“He was the fan you were told about when you were younger. Everyone had a story about him and the amount of games he had been to."
He added: “Away games will just not be the same again - obviously the same with Liam."
A worker at the Newcastle United memorabilia shop close to St James' Park added: "John must hold the record for the most games ever attended by a fan. I remember him boasting that he hasn't missed a Newcastle match since 1973.
"That's pretty good for a man in his sixties.”
Andrew Phillips, 21, was a friend of Mr Sweeney and said he was a really good kid and was known as a very committed fan.
'REALLY GOOD KID': Liam Sweeney
"I've known Liam for about three or four years, we were good mates - we used to go to the away games together," he said.
"He was a spot on kid, no bother at all, it's such a sad day.
"John hadn't missed a game in so many years and Liam was building up a reputation exactly like John going to all the games.
"It's totally devastating to find out what's happened to him.
"I'm sure he'll be looking down on us and will be proud that he has a memorial on Sir Bobby Robson's statue.
"He was no harm at all just a really good kid, it's a shocking day."