Some fans and politicians were angry that poppies were not worn by players.
In a statement, Ulster Rugby said several other Pro12 clubs had not worn a poppy during games at the weekend.
It said its players had observed a minute's silence and it would hold a memorial service on Tuesday.
The Belfast Telegraph reported that some fans had described the absence as "shameful".
Robert McCartney, a former soldier who runs the Beyond the Battlefield charity, told BBC Radio Ulster's Talkback programme it was "an insult".
"I honestly believe they did cause an insult yesterday," he said.
"It is a mark of respect, the reason they are able to play the sport themselves is because of the freedom these men gave them and if they can't put the poppy on their shirt for one day and show that respect, then they really did disgrace themselves yesterday.
"We lost an awful lot of people in the wars and probably members of their family as well and for them not to wear it was a complete insult to the country."
Doug Beattie, an Ulster Unionist councillor and former army captain, said he believed remembrance was a "personal thing".
"The Ulster Rugby team had a remembrance service on that day, they had a minute's silence before the match and everybody is complaining about the symbolism of wearing a poppy, well I just don't get it," he added.
"I don't like the word poppy fascism or poppy Stalinism, there are people who are passionate about the poppy and remembrance and me and Robert have got more in common than we don't, but on this issue I feel that people should wear the poppy if they want to wear the poppy if they don't then they should not.
"I would rather stand beside somebody who is wearing it for a reason than somebody who is wearing it because somebody has told them if you don't you are unpatriotic."
The Ulster Rugby statement said "players and supporters respectfully observed a minute's silence prior to the fixture against Newport Gwent Dragons on Sunday".
"Kingspan Stadium has a permanent War Memorial Arch, which pays respect to those fallen during WW1 and WW2," it added.
"Each year, Ulster Rugby pays respect in its usual and traditional manner with a memorial service at the arch, during which a wreath is laid by the Ulster Rugby President."
The statement added that Newport Gwent Dragons and Cardiff Blues were the only Pro12 clubs who wore a poppy on their sleeves at the weekend.
Ulster Rugby has responded to criticism after poppies were absent from players' jerseys during their Pro12 game against Newport Gwent Dragons on Remembrance Sunday.