Mob life in general was violent in the 20th century, so it's no surprise that many mobsters died young. In The Irishman, though, there does seem to be a cluster of deaths in the period you mention which might appear to indicate that this was a particularly violent period in mob history, but that might be an artifact of who the film is looking at rather than bigger trends in organized crime.

In the late 1970s/early 1980s, organized crime in Philadelphia--which was Frank Sheeran's home base--was roiled by attempts by New York mobsters to muscle Philadelphia out of its traditional position as overlord of Atlantic City. After decades of decline, Atlantic City was suddenly a hot property with the legalization of casino gaming there in 1976. This accounts for the deaths of Angelo Bruno (1980) and Phil "Chicken Man" Testa (1981), the successive heads of the Philadelphia mob. If you wanted to continue to focus on Philadelphia, Testa's son Salvatore was killed in 1984. Altogether, about two dozen mobsters were killed in the space of about five years in the early 1980s (Sifakis 1987, 318-19).

There was also violence around influence over the Teamsters Union, particularly its pension fund, which the movie does a good job of explaining, as Sheeran was central to this. Allen Dorfman was killed (allegedly) because others in the Chicago/Kansas City mob were concerned that he might cooperate with the Justice Department following his conviction on federal bribery charges.

Interestingly, the movie puts Dorfman's death in 1979, when in real life he was killed in early 1983. I'm not sure why that was changed in the movie, but it certainly does make it look like there is an upsurge in violence over those three years. But, depending on the story you were telling, you could probably find similar clusters in other periods as well.