I always took this to mean, 'I forgot'. I understand that it can also mean,'I made a mistake'. Do you have to completely rely on context to know which is intended or is there a difference is its usage?

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dearmad a dhéanamh ar rud "to forget sth"
dearmad a dhéanamh i rud "to make a mistake in sth"

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

This I was familiar with.
This I have never used. I just used: 'rinne mé botún' although have not had much occasion to use the particular expression......

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I'm not completely sure of all the possible ways of saying it but -

Rinne mé dearmad. - I made a mistake/I forgot it.
Rinne mé dearmad air. - is always =I forgot it.
Context will tell.
I think "dearmad" is used more in Conamara than elsewhere.

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It is recommended that you always wait for three to agree on a translation.
I speak Connemara Irish, and my input will often reflect that.
I will do an mp3 file on request for short translations.
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I think it is disguised as dearúd/dearmhad in Munster?

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

,

What's also common among native speakers in Munster is treating dearmhad/ dearúd/ dearmad as a verb; so instead of saying Dhein mé* / Dheineas dearúd, you say Dhearúdas.

* The "é" on "mé" isn't a strong "é" in Munster; that's why "mé" is often written "me" and "tú" as "tu") in older Munster literature.

I would think "botún" is used more widely for mistake, though "dearúd" can be heard also. It makes sense for me that "dearúd" would also mean "mistake" because essentially most mistakes are caused by forgetfulness.

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Please wait for corrections/ more input from other forum members before acting on advice