Roland is the central character in a classical Old French epic tale, The Song of Roland, about Charlemagne that is dated possibly as early as the eleventh century (1000s). One translation, by Charles Scott Moncrief begins:
Charles the King, our Lord and Sovereign,
Full seven years hath sojourned in Spain,
Conquered the land, and won the western main,
In the epic, Ganelon commits a crime and betrays Roland who is martyred (slain) and Charlemagne (also called King Charles the Great) avenges Roland's death. So a critical part to the epic is Roland's death, after which he’d appear--if he appeared--as a ghost.
The allusion in "Mr. Flood's Party" to The Song of Roland calls up the image of Roland's ghost: "Like Roland's ghost." The whole allusion reads:
Alone, as if enduring to the end
A valiant armor of scarred hopes outworn,
He stood there in the middle of the road
Like Roland's ghost winding a silent horn.
In the epic, Roland himself was alone in the moment of his martyrdom. He lifted an ivory horn to use it's sound to call Charlemagne to his side to rescue him. Now, Mr. Flood stands as young Roland stood, in "valiant armor of scarred hopes outworn," and symbolically raised his jug to call his beloved ones to his side. The reply he receives from those who in "other days had honored him" is "A phantom salutation of the dead" friends long ago lost to time and death. Roland's horn is silent because he is dead; Flood's "horn" is silent because his friends are dead.
You can see, now that you understand the allusion to the Old French classic, that the allusion to Roland means that in "Mr. Flood's Party," Roland is the name of a young knight who served and was, in the epic tale, avenged by Charlemagne, King Charles the Great, King of the Franks.