Ashley Grace Irwin was arrested in April on manslaughter charges.
An investigation revealed the infant died in 2018 from a lack of food and medical care.
Irwin pleaded guilty Friday afternoon to a charge of criminally negligent homicide.
As part of the plea agreement, Irwin will testify at the trial of co-defendant Douglas Dale Johnston, the father of the child.
Johnston was arrested earlier this year in Alaska after neighbors there learned he had a warrant out of Oregon.
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