Intentionally boring - exactly as planned Many people do compare this movie to Goodfellas or Casino - and describe The Irishman as too boring. Actually I do think this is exactly what Scorsese intended.
The life of crime is boring and kinda pointless. In Goodfellas the main hero ends as a anonymous guy in witness protection program. In Casino the main hero ends as a old gambler. Frank Sheeran is way more successful: He survives everyone. In Casino there is a bunch of old mafioso guys who are literally judged on wheelchairs. This is their story!
Sheeran is fascinating figure. He does not give a damn about murder. He kills fast and very efficiently. This is not torture, cruelty, example execution, he is much more efficient. This is both boring and frightening, because... it is his job. He is absolutely OK with it, has not any problems with that except maybe doubt here and there and, of course, his daughters are frightened by him and they try to avoid him like a plague.
And there is the last part. This is the most boring and the most powerful one. He survives everyone - but there is no redemption, no lessons learned, no need to say the truth or to seek mercy of the God. He tries to speak with his daughter - and fails. They as him to say the truth as everyone is dead already, but he is silent. He sees no need for pardon from God.
He just buys his coffin and the grave site. He is alone and he is kinda calm and okay with it. His life had no meaning, there is just plain boredom and waiting for the death. There is no glory being mafioso hitman. This is the most tense ending of all Scorsese movies, as there is no glory at all. There is no glory even in the beginning - because for Sheeran, murder is just a job, very similar to work in the factory.
Showing that the Scorsese tells the real truth of being mafia man: You just listen for orders, execute the will of others, kill quickly, avoid problems - and if you are ultra successful, you will end up alone, just calmly waiting for your death, because there is no fun or glory in this job. Chilling, really.