The 34-year-old has put pen-to-paper on a one-year contract at the Jakemans Community Stadium after leaving National League rivals Hartlepool United.
Holohan has previously played under Paul Hurst and Chris Doig at Grimsby Town – and won the National League promotion final in 2022, having done the same with Hartlepool the year before.
Holohan will officially link up with the Pilgrims upon expiry of his contract at Victoria Park on June 30.