He attended Holy Name Parochial School.
[1] Kirnan was a member of the New York State Assembly (Kings Co., 7th D.) in 1931, 1932, 1933, 1934, 1935, 1936, 1937, 1938, 1939–40 and 1941.
He resigned his seat on January 31, 1941, to run for the State Senate seat vacated by the death of John J. Howard.
Kirnan was elected on March 11, 1941, to the New York State Senate, and remained in the Senate until 1946, sitting in the 163rd, 164th (both 5th D.) and 165th New York State Legislatures (13th D.).
He died on January 30, 1948, at his home at 516 Seventeenth Street in Brooklyn.