"Ba" is the past tense of "Is".

It was a dog.
Ba mhadra é.

It causes a séimhiú:

Ba gharda é.
Ba bhean í.
Ba mhúinteoir é.
Ba mhaith liom subh.

Before a vowel it becomes "b'":

B'fhéidir liom snámh.
B'asal é.
B'aoibhinn liom dul go Luimneach.
B'ait an cailín í.

Except when you have "é", "í", "iad" (or their emphatic forms):

Ba é Seán an ceannaire.
Ba í Máire an mháthair.
Ba iad Niamh agus Pól na páistí.

I want to clarify when "ba" becomes "ab". I was taught the following at school:

Seán is the oldest boy in the class.
Is é Seán an buachaill is sine sa rang.

Seán was the oldest boy in the class, before Pól came.
Ba é Seán an buachaill ba shine sa rang, roimh theacht do Phóil.

Máire is the most beautiful girl in the class.
Is í Máire an cailín is áille sa rang.

Máire was the most beautiful girl in the class, before Sinéad came.
Ba í Máire an cailín ab áille sa rang, roimh theacht do Shinéad.

Does every dialect use "ab" in the above sentence?

What about in the following:

Seán is ainm do.
Seán ab ainm dó.

Does every dialect use "ab" in the above sentence?

Feirmeoir is ea é Seán
Feirmeoir ab ea é Seán.

I've seen many written examples of "Feirmeoir ba ea é Seán", so I presume this is dialectal?

Before a vowel, you have "b'" in direct speech, and "ab" in indirect speech. Is this correct?

One thing though, I notice that when you want to express "taller, bigger, more beautiful", you use "b'" instead of "ab":


Bhí Máire go hálainn.
Ba í Máire an cailín ab áille sa rang.
Bhí Máire ní b'álainn ná Sinéad. (rather than "ní ab álainn")

Is all of the above right?

I see that some dialects are more particular about when they séimhiú after "ba", and also about when they turn "gur" into "gurb". Would many dialects say:

Deir sé gurb amadán é.

Or would more say:

Deir sé gur amadán é.

Finally, is there any particular circumstances in which people use "go mba" instead of "gur, gurbh" or are they equivalent?

Seán is ainm do.
Seán ab ainm dó.

Does every dialect use "ab" in the above sentence?

Donegal dialect does.

Feirmeoir is ea é Seán
Feirmeoir ab ea é Seán.

I have learnt that this kind of sentence is typical of Munster dialect.

Wouldn't the second examples be questions, just using different word order?

Ba Máire an cailín (ab áille?) sa rang.

>>Wouldn't the second examples be questions, just using different word order?

They are relative clauses.

Ba Máire an cailín (ab áille?) sa rang.

The right sentence would be "Ba í Máire an cailín ab áille sa rang."

Aw yes, I've found that now guys. Grma. "Comparitive degrees of adjectives with past tenses or conditional".

"When a sentence is in the past tense or conditional,ní ba frequently replaces níos in a comparison, and ba replaces is in the superlative."

- Ó Siadhail
Learning Irish Ch 32.6.