A chairde,

I have been learning Irish for a while now and I have come cross several different ways of saying "for" as gaeilge.

do, le haghaidh, go ceann, ar feadh, le,

I could understand the differences between most of them but baffled with 2 in particular, "le" and "ar feadh".

If I want to say "I have lived here for 5 years", Táim i mo chonaí anseo le/ar feadh 5 bliana.

Can I use either of them? Are they interchangeable? Or are there any rules for using them to talk about the past/perfect tense?

Mile Buiochas!

Song

Tá mé i mo chónaí anseo le 5 bliana (anuas) (I've lived here...)

I don't think you can use "ar feadh" with the present tense, you'd rather use it with the past:
Bhí mé i mo chónaí anseo ar feadh 5 bliana. (I lived here...)

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

Hi Lughaidh,

I think you are absolutely right! So if I understand correctly:

"go cean" is for the future
"le" is for the present/present perfect
"ar fardh" is for the past
"le haghaidh" is for reason/things
"do" is for people

Go raibh mile maith agata!

Song

Song先生、欢迎！欢迎！Fáilte go dtí ár bhfóram.

I think you can use ar feadh for any period in the past, present or future, provided that it doesn't start or end now.

Bhí mé in Éirinn ar feadh trí seachtaine. "I was in Ireland for three weeks." (altogether, starting and ending in the past.)
*Tá mé in Éirinn ar feadh trí seachtaine. "I am in Ireland for three weeks." (altogether, starting in the past or ending in the future).
Beidh mé in Éirinn ar feadh trí seachtaine. "I will be in Ireland for three weeks." (altogether, starting and ending in the future)."

If you use le, it implies you have been here already for that period:

Tá mé in Éirinn le trí seachtaine anuas. "I have been in Ireland for the past three weeks." (so far, until now)
Beidh mé in Éirinn go ceann trí seachtaine. "I will be in Ireland for the next three weeks." (counting from now or very soon in the future)

(Grammar check please.)

Some confusion also arises because Hiberno-English uses "I am" where standard English uses "I have been". If an Irish person says "I am here three weeks now", they usually mean that they have been here for three weeks already, whereas when non-Irish people say "I am here for three weeks" they are usually expressing the total length of their stay.

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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).

I don't know that. I'd say "tá mé in Éirinn go cionn X seachtaine".
Otherwise I'd say "caithfidh mé 3 seachtaine in Éirinn" (I'll spend 3 weeks in Ireland)...

Btw it's "in Éirinn" (slender nn) and not "Éireann".

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

It may be that people avoid this particular construction (*Tá mé in Éireann ar feadh trí seachtaine) because of similar problems in Irish to the abovementioned confusion in English, and use phrasing like Beidh mé in Éirinn trí seachtaine ar fad. "I'll be in Ireland (for) three weeks altogether", etc., instead.

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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).

Caithfidh mé trí sheachtain... nach ea?

Niet, "seachtain" is one of the few nouns that have a specific (mandatory) plural after numbers from 3 to 10 (and plurals aren't lenited after the leniting numbers). Have to say:

(aon) seachtain amháin
dhá sheachtain
trí seachtaine
ceithre seachtaine
cúig seachtaine
sé seachtaine
seacht seachtaine
ocht seachtaine
naoi seachtaine
deich seachtaine

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

Tá an ceart agat... GRMA Nílim ag labhairt go minic a thuilleadh, agus mar sin tá roinnt mheasartha den líofacht (na rudaí "beaga tábhachta" seo) caillte agam GRMA

Bhail cha minic a labhramsa fosta, ach scríobham achan lá

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