Ara, ná bac leis - - Oh, do not heed him

Ara - - an exclamation of disagreement or contempt

The Ó Dónaill Dictionary gives Ná bac é - - do not heed him
and Ná bac leis - - do not interfere with him. The distinction should be made but, sadly, this is a living phrase in common use in the schoolyard where subtlety of expression is frequently lacking.

This is an ordú diúltach - - negative command - - (You would never use the direct positive command of this verb other than humorously)

To give a negative command simply put ná before the verb making no change before a consonant: ná déan deifir - - do not hurry; but put a h before a vowel: ná héist - - do not listen

Éist - - listen; éistígí - - listen (plural); ná héist - - do not listen
Gabh - - accept; gabhaigí - - accept (pl) ná gabh - - do not accept
Múch - - quench; múchaigí - - quench (pl); ná múchaigí - - do not quench