This is something that I noted down sometime ago, but I just couldn't find it, but now I have - it was provided by Redwolf.

One of the copula rules is that a definite noun can't come immediately after the copula - there has to be a pronoun there.
So for example:

Is athair é - He is a father
Is é m'athair - He is my father
Is sagart é - He is a priest
Is é an sagart - He is the priest

In Connacht, I think those would be:

Is athair é - He is a father
Is é m'athair é - He is my father
Is sagart é - He is a priest
Is é an sagart é - He is the priest

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WARNING: Intermediate speaker - await further opinions, corrections and adjustments before acting on my advice.
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect.
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), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

I think it would be the same in Munster - so what is given there would be in Standard?

I think the Standard just says the final pronoun is optional - meaning you can do it the Ulster way (without the final pronoun) or the Connacht/Munster way (with the pronoun).

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WARNING: Intermediate speaker - await further opinions, corrections and adjustments before acting on my advice.
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect.
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), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

I think you'd have
"Is é m'athair é"
"Is í mo mháthair í"

in all dialects and in the CO too...

to me these sentences are wrong if you don't have "é/í" at the end.

The difference between Ulster and the rest is in sentences like:
Is fear Liam (while C and M: Is fear é Liam).

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

'Sé mo laoch, mo ghile mear...

Is breac é.
Is bradán é.
Is é an bradán é
Is iasc é an bradán.
Is breac mór é.
Is mór an bradán é.
Is é mo bhreac mo bhradán mór.

isn't it "Is é mo laoch mo ghile mear" = my spirited brightness is my hero ?

it would explain why there's no "é" at the end...

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

Yes.

But just to make it clear to people. If there is more coming after it then the second "é" is not included.
Is é m'athair a rinne an obair sin.

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WARNING: Intermediate speaker - await further opinions, corrections and adjustments before acting on my advice.
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect.
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), agus Gaeilge an Chaighdeáin Oifigiúil (CO).