Brown joined Sunderland from Man Utd in July 2011 on a four-year-deal for an undisclosed fee but saw his first season for the club cut short when he was injured during the FA Cup third round tie with Middlesbrough in January 2012.
Martin O'Neill: "It's been a long time out for Wes, who has been beset with injuries.
"He looked as if he was just settling at the time I came in, but hurt himself in January last year and we've not really seen him back since.
"He had a setback out in Korea but it looked like he was making progress, but for Wes it's been a case of two steps forward and three back.
"Chances are we probably won't see him between now and the end of the season, but perhaps it'll be one he wants to forget and start again next season.
"He's joined in training on a few days and then has to take some time out again.
"At the moment, however, I think he's reversing that trend - it's one little step at a time at the moment."