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Irus was a beggar and an errand boy who was also an incorrigible glutton and drunkard. He picks a fight with Odysseus because he feels Odysseus is competition as a beggar.
Odysseus was on his "terf" and irus didnt like the competition.
Odysseus won because Athena had gven him extra strength
they started betting
The suitors encouraged the two to fight. They began wagering on the outcome, and shouted at the participants to beat each other down. They also swore not to give Odysseus a dirty blow while they were fighting. Antinous threatened Irus should he lose.
Basically, Odysseus is on what Iros considered to be, 'his turf' and does not appreciate the competition. Odysseus easily defeats Iros.
Irus was the only thing that stood between him and his home, which was lousy with suitors. Irus would run messages for them and to keep up appearances, he had to dispatch Irus without letting on who he was.
Irus quarrels with the stanger because he comes to have second thoughts about the wisdom of a proposed match against Odysseus.
Irus believes that he being the younger will be able to get rid of Odysseus entirely, and thus have no more competition begging at the suitors' feast.