FX ordered 13 more episodes of the critically acclaimed series, which stars Leary as an alcoholic New York City firefighter struggling to cope in the aftermath of the Sept. 11, 2001, terrorist attacks.
Leary is the series' co-creator and co-writer, along with Peter Tolan.
"Once again, Denis and Peter have delivered a brilliant season," FX Networks President and General Manager John Landgraf said in a statement. "They continue to create wholly original storylines, which pull off the difficult task of blending riveting drama with raucous comedy. We're extremely proud of this show, which is one of the very best television has to offer."
The second season finale is scheduled to air Sept. 13.
Earlier this year, Leary and Tolan were nominated for an Emmy Award for Outstanding Writing for a Drama Series and Tolan was nominated for Outstanding Directing for a Drama Series.