The Night Fighters makes use of Robert Mitchum's marvelous gift for speech mimicry as he adopts and Irish brogue for a film set in Northern Ireland in 1941. The brogue would be put to better use later on in a far better film, Ryan's Daughter.
To put in historical context, France has fallen and the British are waging war without allies on the continent for a year now. The Germans contact elements of the old Irish Republican Army and they're looking for a rising like Easter of 1916 that will drain British troops to defend the country and free the six counties who were left behind.
So the local commandant Dan O'Herlihy raises such a force in his local area and recruits buddies Robert Mitchum and Richard Harris for the group. They do a raid and one of their number is killed and Mitchum and Harris are fugitives. All the worse because as history records the Nazis never did attempt an invasion of Great Britain, they went to the Soviet Union instead.
Mitchum and Harris are a pair who would rather raise a tankard of beer than a country see this was for nothing. That leads to all kinds of complications down the road.
I'm not sure what the point to The Night Fighters really was. The film ending was anti-climatic to say the least. I will say that Mitchum's character had a lot of Victor McLaglen's Gypo Nolan from The Informer in him. The Night Fighters never reaches to the dramatic heights that the John Ford classic did.
In fact The Night Fighters is known primarily for the fact that after hours Robert Mitchum got into a pub brawl with some of the locals and he came out on top. Many different versions of the story got into the media around the world, but I suggest you read Lee Server's meticulously researched and delightfully written biography of Mitchum to get the real story. I can't repeat it here due to the language.
Mitchum fans might like The Night Fighters, but it will never be at the top of his best screen performances.