As for Boromir/Cuchulain questions, similar death scenes can be found also elsewhere. For instance, Henryk Sienkiewicz, a Polish writer and Nobel laureate for the novel "Quo Vadis", also wrote a novel "With fire and sword", about a Polish-Ukrainian war taking place in seventeenth century. In this novel one of the heroes, a valiant and noble although somewhat simple Lithuanian, Longinus Podbipieta, volunteers to make an attempt to save his companions besieged in the fortress of Zbaraz by trying to escape in secret their besieged camp and to bring news about their desperate need to the king and his army. However, he fails and he dies in a very similar fashion as Boromir, pierced by many arrows.
Did Tolkien read that novel? I do not know, but I know that he had Polish acquaintainces, who might have recommended that book to him. He was a Roman catholic and his grave in Oxfors is surrounded by graves of Poles, emigrants who had to leave Poland during the 2nd World War. We also may remark that in LOTR Aragorn uses the expression "with fire and sword" when talking to Merry in the Houses of Healing:
"Master Meriadoc" said Aragorn "If you think that I have passed through the mountains and the realm of Gondor with fire and sword to bring herbs to a careless soldier who throws away his gear, you are mistaken"
The actual scene of Boromir's death in the arms of Aragorn also bears much resemblance to the scene from a classic adventure novel, "The Prisoner of Zenda", in which Rudolph Rassendyl, severely wounded, is fainting in arms of his friend...