No, in German they are quite different. Let me explain it with an example:
Wenn ich traurig bin, trinke ich manchmal Alkohol.
Sometimes, when I’m sad, I drink alcohol.
Another one:
Irgendwann werde ich das Fahrrad reparieren.
One day I’ll repair the bicycle.
I would translate „irgendwann“ with “eventually”, “one day” or “someday” as it means that something will/might happen in the future (OR has happened in the past; I’m not sure whether this meaning isn’t just colloquial, though). Another example:
Irgendwann stirbt jeder (Mensch).
Eventually, everybody is going to die.
But beware: “eventually” can also mean „letztlich“/„letztendlich“.
The Duden explains it quite well using „zu irgendeinem Zeitpunkt“ as an explanation.
EDIT:
I just saw in your profile that you are from Paris so I assume you speak French? I checked a dictionary German->French:
irgendwann --> « un jour ou l'autre »
manchmal --> « des fois », « parfois », « quelquefois », « à l'occasion », « par moments»
While my French doesn’t go much further than knowing what Je cherche une chambre
means, it might be helpful for you. Unfortunately, I cannot provide you with the information of which translation of „manchmal“ is the right one.
Conclusion:
„irgendwann“ means “at an uncertain point in time/some time”
„manchmal“ means “sometimes”