Hello. I'm studying Irish language.

I want to use Irish phrase "Don't forget me".
I think it is translated to "Ná déan dearmad orm".
Is it natural phrase?

I think "dearmad" is used with "ar".
But do you say so commonly?

If there is general expression, please teach me.

It is: ná dearúid me or ná dein dearúd díom.

In "standard" Irish: Ná dean dearmad díom or Ná dean dearmad orm.
The version given by djwebb is used only in the Munster dialect, which has the fewest speakers.

As Errigal would probably point out, Peadar Ua Laoghaire only commented on his own dialect. He wrote in Notes on Irish Words and Usages (p37):

I'd think there are two different kinds of "ar":

Tá dearmad orm.
is different from
Ná déan dearmad orm.

The construction "déan + (transitive) verbal noun" usually always has ar + (otherwise direct) object, no matter what transitive verbal noun is used:
déan + VN + ar + object

e.g. Déan deisiú ar na bróga, déan léamh ar an leabhar, etc.
And so: Ná déan dearmad orm.
(which is just an altered form of: Ná déan mé a dhearmad. or simple: Ná dearmad mé.)

Ar rather belongs to the (auxilliary) verb déan than to deisiú, dearmad, or léamh.

In case of (in strict sense intransitive) verbal nouns with prepositional objects, of course this preposition is used:
Ná déan dearmad díom.
Because dearmad can be used both ways, both forms with déan (díom / orm) are correct.