I started learning Irish a year ago but struggle with the pronunciation with my surname. I was wondering if anyone had any idea on how to pronounce Céatfhadha ? Especially in a Connemara Irish ?

GRMMA

Ó Céatfhadha
Probably [k´e:tə] as if written Céata

My first guess was /k´e:tə/ too.

But further research suggests the ending may be pronounced -aí /i:/, as if the whole thing were spelled Ó Céataí /o: k´e:ti:/.

Is the English version "Keaty"? Irish Identity gives Ó Céataí for Keaty. Sloinne gives both Ó Céataí and Ó Céatfhadha for Keaty. If so, I'd say it's pronounced /o: k´e:ti:/.

Past participle endings such as -tha are also pronounced /i:/ in Conamara.

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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).

Yes, I'd agree.
The nominative of the name following Ó is Céatfhaidh, pron. already /k´e:ti:/, so genitive -fhadha is just a spelling change without changing the pronunciation.

GRMA An-spéisiúil, agus d'fhoghlaim mé rud as.

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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 was using it for the translation of Keating, do you reckon it'd still be the same anyway ?

Cheers

Yes, the pronunciation of Ó Céatfhadha remains the same.

Sloinne gives Ó Céatfhadha, De Céitinn, and Céitinn for "Keating"

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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).