The rapper jumped on IG after news broke Wednesday that Irv had died ... 50 posted the photo of himself sitting next to the "RIP" scrawled tombstone and exhaling smoke from a hookah.

In the caption, Fitty wrote, "I'm smoking on dat Gotti pack," but then softened his language with "nah God bless him LOL," alongside an image of Irv -- the co-founder of Murder Inc. Records.

The day before, Fitty referenced a news report about Irv being on life support, saying on IG, "Damn, I wanted him to see my new shows," meaning his new FAST Channel.

Obviously, there was no love lost between the two music moguls who had a longstanding beef dating back to 1999, when Fitty's friend was supposedly robbed by Ja Rule -- one of Murder Inc.'s most famous rappers. Irv also produced albums for other major artists ... including Jay-Z, Ashanti, DMX and J Lo.

At the time, Fitty's career was just taking off and he didn't get along with Irv and Ja from the get-go ... there were several clashes -- one occurred in 2000 when Ja and 50 got into a fight at an Atlanta nightclub. 50 and Irv never hugged it out before Irv's passing.

As we reported ... Irv died Wednesday at 54 in New York City ... but his cause of death is still unknown, although he had recently been battling health problems. Last year, Irv suffered a stroke and was forced to walk with a cane. He also struggled with diabetes and tried to change his poor diet per his doctor's orders.

By the way, Irv's record label was the subject of a five-part 2022 BET documentary, "The Murder Inc Story."