Iros (also, Arnaios, the "getter" because he runs errands for the suitors) is the town beggar, described as

. . . not very strong, and had not a trace of courage, but he was a great big hulking fellow to look at. (18:4-5)

Clearly, Iros is a bully and sees Odysseus in his disguise as a old, decrepit beggar, as someone who has to be thrown out because Odysseus is competition, particularly because the practice of xenia (hospitality) in the Bronze Age Greek world compels the suitors and Telemachus to offer food, drink, and temporary lodging to Odysseus as beggar. A second motive for Iros to get rid of Odysseus rests with the suitors themselves, who are apparently giving Iros some encouragement:

"All these men--can't you see--are winking at me and signaling me to haul you out of this house, although I don't want to do that. So go away or these fists of mine will persuade you." (18:12-14)

In addition to ridding himself of Odysseus, Iros threatens Odysseus in response to the suitors, who, by encouraging an attack on a beggar seeking hospitality, are violating the sacred obligation (because the commandment comes from Zeus) of xenia and usurping the right of Telemachus, Odysseus' son, to run his household as he sees fit in his father's (presumed) absence.

Iros' challenge to Odysseus, then, especially if the challenge is encouraged by the suitors in violation of a sacred obligation, places both Iros and the suitors in a very dangerous position--in their violation of hospitality, they become the targets of justified retribution. Iros is immediately struck by Odysseus, not a fatal blow but enough to stop his threat, and the suitors have become legitimate targets of Odysseus' revenge.

In Odyssey 18, we find Odysseus disguised as a beggar inside his own home on Ithaca. His mission is to destroy the 108 suitors who have been harassing his wife Penelope for the past several years. In Book 18, Odysseus wants to test the hospitality of the various suitors, so he goes around and begs from them to see if he will receive hospitable treatment. 

Unfortunately for Odysseus, another beggar named Irus (also spelled Iros) has essentially claimed Odysseus' palace as his "turf" and so Irus does not appreciate the competition from Odysseus. Given this, Irus acts in a hostile way toward Odysseus and challenges Odysseus to a fight, a battle which Odysseus wins. Odysseus then drags Irus out of the palace. 

This event foreshadows Odysseus' destruction of the suitors and results in an ironic blessing from the suitors, who offers Odysseus congratulations for his victory over Irus:

"Stranger, may Zeus and the other gods grant you your heart’s dearest wish, your fondest desire, since you’ve ended that greedy fellow’s begging here." (A.S. Kline translation)

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