This week the rather unlikely terms ‘Castle Catholic’ and ‘West Brit’ trended on Twitter.
The reemergence of the two phrases associated with the beginning of the 20th century sparked much debate online.
Historian Donal Fallon explained how the terms have been around surprisingly longer than we might expect.
He joined On The Record with Gavan Reilly for an episode of Hidden Histories to explain how the discourse was used in the past.
He said he "couldn't believe it" when he logged on to Twitter this week and saw that 'Castle Catholic' was trending.
Mr Fallon explained: "It's a really nasty term than in comparison to 'West Brit' and it was kind of a much more serious charge to level against someone."
He said the term implies that someone is "not really Irish" or they are "the enemy within".
This is due in part to Dublin Castle being the centre of the British administration when Ireland was under English rule.
The term also has social connotations and indicates social aspiration due to the association with a castle.
Mr Fallon said: "A Castle Catholic is basically perceived to be a clog in this great, big British bureaucratic machine."
It symbolised "an enormous fortified institution" with gates that most Irish people would never have got through.
He added that there is a "whole host of terms" that come from Dublin Castle in the early 20th century, such as informer.
"It's a term that lingers on through the Troubles", he continued, with John Hume writing of being fearful of being labelled with that name during the peace process.
Mr Fallon said: "Calling someone a Castle Catholic is like saying they're part of this great political conspiracy.
"There's a sharp edge to Castle Catholic that isn't really there in West Brit.
He added that there is "a really dangerous conspiratorial aspect" to the term too as it implies that such a person would have a "colonised mind" and would try to undermine the Irish nation.
In comparison, a 'West Brit' comes from the Act of Union in the 1800s and that Ireland was no longer seen as a notion but an "outpost" of Britain to its west.
He said that the term gathered greater prominence in Ireland in the early 20th century, with radio host Terry Wogan later reviving the term after describing himself as a 'West Brit" for listening to the BBC rather than Irish radio stations when growing up.