The actor (31) plays rebellious Tom Branson in the hit period drama and had many of the biggest storylines in the third series.
However, eyebrows were raised among Irish viewers when a recent episode showed Maggie Smith, who plays the Countess of Grantham, referring to Branson's Irish brother as a "drunken gorilla".
And there is no shortage of anti-Irish sentiment in recent episodes, given the aristocratic family's horror at his decision to raise his baby with his wife Lady Sybil as a Catholic.
Yet Allen, in Dublin to launch Brown Thomas's Christmas season, said it was a realistic portrayal of the era, given all the Anglo-Irish tensions arising from the 1920 War of Independence.
"You have to remember that, at that time, in England, that's actually how the Irish were depicted," he explained.
"Ireland is pretty much to the fore in this series. I think it opens a dialogue in relation to what actually did happen and makes people realise what Ireland was going through at the time."
He added: "I think Branson shows how passionate people can be in relation to their own country and he's passionate about everything he does, be it in going after a woman or his pleas for his country.
"It's great that it brings Ireland's history to a whole new audience."
The former Man About Dog star went on to explain how he had to hit the history books to make sure it was a genuine portrayal of his character, who started off in the series as a chauffeur.
"It was fascinating going back through it all," he continued.
"A lot of people did travel over, maybe not with his political ideology, but they would have gone anyway."
The young actor's star is on the rise thanks to the success of the show, watched regularly by 10m viewers.
But he hasn't forgotten his roots -- he will be back with his family in Killiney over Christmas.