Answer:
The repetition of the lines "Brennan on the Moor, Brennan on the Moor, Bold and undaunted stood young Brennan on the Moor" in the poem emphasizes the fearless nature of William Brennan by creating a sense of rhythm and urgency in the face of adversity.
First, it underscores Brennan's unyielding demeanor as an outlaw who robs the rich and divides his spoils with widows in distress. The repetition reinforces this idea by portraying his fearlessness in taking on opposition from those who are wealthy and powerful, which are central to his character and actions throughout the poem.
In the end, the phrase repeats again in the honor of his bravery by facing death itself as he bids farewell to his family. This reinforces not only loyalty, but as well as his unyielding determination to stand up for his principles and protect those he cares about.
Ultimately, the author is able to brilliantly make use of repetition by adding depth and intensity to the character of William Brennan -- unafraid to challenge authority and to face the consequences of his actions --, elevating him as a legendary figure in the narrative.