Before the images were shown, Mr Kielty remarked that the reality programme was “full of deception, betrayal, everyone keeps changing their story as well. All we have to work out is who’s telling the truth”.
He then used the term “Sinn Féin traitors” which the party said is “a slogan used by far-right activists in Ireland.
In a statement, Sinn Féin’s director of communications Robbie Smyth said: “Mary Lou [McDonald] has herself been subjected to death threats, where the moniker ‘traitor’ has been used”.
He went on to say “repeating far right tropes endangers our party members and elected representatives who have been harassed by these groups”.
However, RTÉ said that “under Patrick Kielty’s tenure, The Late Late Show has become more satirical and this is evident with an opening monologue based broadly around topical issues”.
In a statement, the broadcaster said “the presenter joked about recruitment for the new television series ‘Traitors’ which is based on a group of people facing, and being tested by, various challenges”.
“That was the satirical context, given the recent controversies involving Sinn Féin.”
The broadcaster then pointed to “an implicit joke about the House of Windsor” which immediately followed the remarks, which RTÉ said “further reinforced” the “satirical aspect of the section”.
RTÉ has also reported that Sinn Féin wrote to the broadcaster’s director general Kevin Bakhurst asking that Kielty “issue a public apology on air next week”.