Fahey flew home to Dublin on compassionate grounds in November but made his first appearance since October 20 against Middlesbrough, helping the Blues record back-to-back league victories for only the second time this season.
After spending most of the campaign looking over their shoulders towards the relegation places, Clark's men are now heading in the opposite direction, with Fahey set to play an important part in their remaining eight games.
Clark told the Birmingham Mail: "Considering he has not played competitively for so long and to perform like he did, it was outstanding.
"It's definitely a big plus to have Keith back in the group.
"He was a very important player for this club last season and we're just happy that he's available again and involved."