The move comes after the Braves released veteran left-hander James Russell in a surprise move last week.
The 25-year-old McKirahan was claimed by the Marlins as a Rule 5 pick from the Chicago Cubs in December. He must remain on the Braves' 25-man roster all season or be offered back to the Cubs.
McKirahan posted 10 saves and a 2.08 ERA in 44 games last season with Class A Daytona and Double-A Tennessee. He has a 2.16 ERA in four minor league seasons. He missed parts of two seasons following Tommy John surgery.
He is expected to report to the Braves' camp on Thursday.