It may have been; but from the first and fourth verses of Messe ocus Pangur Bán:

First Verse

Messe ocus Pangur bán,
cechtar nathar fria saindán;
bíth a menma-sam fri seilgg,
mu menma céin im saincheirdd

4th Verse:

Gnáth-huaraibh ar greassaib gal
glenaid luch ina lín-sam;
os me, du-fuit im lín chéin
dliged ndoraid cu ndronchéill.

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Now THAT's interesting.

So the f in féin is essentially a defective ch anyway. I love it.

In that case, perhaps we should be spelling héin => chéin 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).

both céin and féin (and variants: fadéin etc) existed in Old Irish, but how could we say f is a defective ch?

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

The same way that cluiche can be cluife or thríd can be fríd.

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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).

cá bhfios duit an rabh sin le fáilt i Sean-Ghaeilg cheana féin agus sa chás sin go speisealta?...

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

Cá bhfios dhuit nach raibh?

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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).

Ní fios, agus sin é an fáth nach dtig "So the f in féin is essentially a defective ch anyway" a scríobh.

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

It is more likely to be the case that the f degenerated from the ch than the reverse.

Show me a proven case where a ch arose from an f and I might believe you, otherwise, on your bike with you.

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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).

But what tells you that one of these consonants comes from the other? They might come from a different prefix or something, just as the prefixes do- and so- for instance (sorcha/dorcha, so-dhéanta/do-dhéanta, etc... does it mean that s comes from d or vice-versa? it doesn't).
Old Irish was pretty phonetic in its spelling, and I think if the scribes wrote f, they would pronounce f, not h nor slender ch. And slender ch and f are too different to be related, let alone c and f.

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

If you say chéin for long enough it ends up like sounding like an fh because of the strong "éin" termination. As you said Lughaidh Old Irish was phonetic could Féin, céin, chéin just be phonetic representations of each other and not necessarily different words that have the same meaning?.

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