The Irishman, restricted to 13 first-team appearances for Sean O'Driscoll's side this season, will join the Pirates for an initial month.
Heffernan spent a month on loan at Oldham at the end of last year, scoring one goal in four appearances.
The 28-year-old will go straight into the Pirates' squad for the home clash with Walsall on Tuesday if he is registered in time.
Heffernan joined Doncaster in June 2005 from Bristol City, where he scored four goals in 27 appearances, after making a name for himself at Notts County.
Source: PA
Source: PA