Irma, Part-Time Mutant
At the beginning of combat on your turn, Irma becomes a copy of up to one other target creature you control, except her name is Irma, Part-Time Mutant and she has this ability. Then put a +1/+1 counter on her.
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Official rulings (5) · Scryfall
You don't have to choose a target for Irma's ability. However, if you do and the target creature is an illegal target as Irma's ability tries to resolve, it won't resolve and none of its effects will happen. Irma won't become a copy of anything else, and you won't put a +1/+1 counter on her.
Irma copies exactly what was printed on the original creature and nothing else, with the listed exceptions (unless that creature is copying something else; see below). She doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, and so on.
Because Irma isn't entering the battlefield when she becomes a copy of another creature, any "When [this creature] enters" or "[This creature] enters with" abilities of the copied creature won't apply.
If the copied creature has {X} in its mana cost, X is 0.
If the copied creature is copying something else, then Irma becomes a copy of whatever that creature copied (with the listed exceptions).