Hi guys,
I am currntly studying Irish and i am struggling with the difference between á dhéanamh and a dhéanamh. When do i use á instead of a?
Thanks!!

*currently

á dhéanamh means "at doing it" (literally, "at its doing").
a dhéanamh means "to do" most of the time, but it depends on the sentence. If you have specific examples, it'll be easier to explain that to you. The way the infinitive and the -ing form of verbs are used in English has nothing to do with the way the verbal noun is used in Irish...

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

a < do = lit.: "to"
á = do/ag + possessive adjective a = lit.: "at/to his/her/their"

There are versions of progressive constructions (in normal active main clauses ag) using a or á:
Tá mé ag déanamh rud éigin. = I am doing something
rud éigin atá mé a dhéanamh. = something that I am doing (relative clause, object as antecedent, a instead of ag)
Cad/céard atá mé a dhéanamh? = What (is it that) I am doing? (relative clause, cad as antecedent)
Tá mé á dhéanamh. = I am doing it/him. (pronoun as object, á instead of ag)
Tá mé á déanamh. = I am doing it/her.
Tá mé á ndéanamh. = I am doing them.
Tá rud éigin á dhéanamh agam. = Something is being done by me. (passive)
rud éigin atá á dhéanamh agam. = Something that is being done by me. (passive, relative)
Cad atá á dhéanamh agam? = What (is it that) is being done by me? (passive relative)

And there are infinite constructions using a or á:
rud éigin a dhéanamh = to do something (normal infinitive-like construction with object)
é/í/iad a dhéanamh = to do it/him/her/them (same as above, pronoun as object)
a (n)d(h)éanamh = to do it/him/her/them (variant of the latter, here: a = "his/her/their" instead of "to" depending on (non)lenition/eclipsis of déanamh;
a dhéanamh = é a dhéanamh, a déanamh = í a dhéanamh, a ndéanamh = iad a dhéanamh)
rud éigin (a bheith) á dhéanamh = something (to be) being done (passive progressive infinitive)

Nicely done!