During the first season it was believed Brian Fortune was playing Marsh, and Dennis McKeever was playing Othell Yarwyck, based on comments made by Fortune and McKeever's character delivering one of Yarwyck's lines from the books. However, this assumption was then dispelled: McKeever reappeared in Seasons 2 and 3 in the Great Ranging, even though Yarwyck didn't participate in the books; in season four, Fortune's character was identified as Yarwyck; and Michael Condron was subsequently cast as Marsh for the fifth season, which means Fortune's character was always Yarwyck, while McKeever was simply playing an unnamed Night's Watch officer.
In the books
In the A Song of Ice and Fire novels, Marsh, nicknamed "the Old Pomegranate," is an important and senior figure in the Watch, reporting directly to Lord Commander Jeor Mormont and is just below him in the command hierarchy of the Watch. When Mormont and the rest of the Night's Watch ride out to investigate the wildling menace in the so-called "Great Ranging", he remains in command at Castle Black. Though not explicitly stated, Bowen Marsh hails from House Marsh, a noble house from the Neck sworn to House Reed.
As the Wildlings advance to Castle Black, Mance Rayder sends raiding parties the to attack several areas of the Wall, in order to draw attention away from the main host and to draw away the defenders. Unfortunately, Marsh falls for the trick and takes most of the garrison from Castle Black. When Jon Snow returns, he finds that only forty-one men are left in the castle, most of them old, crippled, and inexperienced.