
irgendwann
someday
At an unspecified point in the future.
Irgendwann möchte ich nach Japan reisen.
Someday I would like to travel to Japan.
sometime / someday
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someday
At an unspecified point in the future.
Irgendwann möchte ich nach Japan reisen.
Someday I would like to travel to Japan.

sometime
At an unspecified point in the past.
Er hat das irgendwann mal erwähnt, aber ich weiß nicht mehr genau, wann.
He mentioned that sometime, but I don't remember exactly when.
The word is made up of 'irgend-' and 'wann' (when). The prefix 'irgend-' expresses indefiniteness, similar to 'irgendwo' (somewhere) or 'irgendwer' (someone). So 'irgendwann' literally means 'at some point in time'.
Ich weiß nicht genau, wann ich Zeit habe, aber irgendwann melde ich mich.
I don't know exactly when I'll have time, but I'll get in touch sometime.
You can tell whether the past or the future is meant from the tense of the verb. The perfect or simple past points to the past, while the present or future points to the future.
Wir haben uns irgendwann in den 90ern kennengelernt.
We met sometime in the 90s.
As an adverb, 'irgendwann' can stand flexibly in the sentence. It can stand in position 1, in which case the verb follows in position 2. But it can also stand after the verb in the middle of the sentence.
Irgendwann versteht man die Regeln.
At some point, you understand the rules.
In colloquial speech, 'irgendwann' is often combined with 'mal': 'irgendwann mal'. This makes the statement even more indefinite and casual.
Wir sollten uns irgendwann mal wieder auf einen Kaffee treffen.
We should meet up for coffee again sometime.
once
Einmal can refer to a single opportunity in the future while leaving open when it will happen. Irgendwann mainly emphasizes that the point in time is unspecified. If the one-time opportunity is important, einmal is appropriate; if only the unknown timing matters, irgendwann is appropriate.

Ich möchte irgendwann nach Wien fahren, weiß aber noch nicht wann.
I’d like to visit Vienna sometime, but I don’t know when yet.
Ich möchte einmal nach Wien fahren und dort ein Wochenende verbringen.
I’d like to visit Vienna one day and spend a weekend there.