I recently read an interesting article by R. A. Breatnach,
An Gléas Teaspáinteach (in
Éigse, vol. XVI, part 3, 1976) explaining the developments of the copular forms like
seo focal le Diarmuid or
seo é an fear. Thought I’d share a summary here, as I understood it.

Maybe others without access to
Éigse would be interested (and also I wanted to have a note here myself).
Classical Gaelic had a set of phrases in the form
ag so X, ag sin X, ag súd X meaning roughly ‘here is X, there is X’ or ‘this is X, that is X’ (I described those briefly in
my WIP notes on Classical Gaelic grammar).
The
X in those phrases could be either in nominative (eg.
ag sin bean gheal ‘that is / there is a bright woman’) or in accusative (
ag sin mhnáoi ngil). In early modern texts the nominative is much more common, but accusative is certainly older (since synchronically it’s not clear
why accusative is used here).
The explanation is that this
ag continues Old Irish (unattested)
*aicc ‘see’ (the imperative of
ad·cí ‘sees’) – doublet of classical
faic (and modern
feic) without the added
f-; the phrase must have been something like
*aicc síu, *aicc sin ‘see here, see there’ in Old Irish, so the
X originally was a direct object of a verb. At some point the literal meaning ‘see’ was removed from the phrase and the verbal stress got weaker, the phrases started to be written with
ag, like the preposition (but there are 14th century examples with
aig and even an example with
ic-seo in the Bk. of Magauran). Both meanings existed in Middle Irish, ‘see, behold, here is’ in
ac-so uan Dé translating Latin
ecce agnus Dei and ‘this is’ visible in
ac-so mo mac dil féin… translating
hic est filius meus dilectus… in the
Passions and Homilies of
An Leabhar Breac.
Then, in the second half of the 16th century the
ag part started to be dropped, and it seems leaving
ag out was the norm in spoken 17th c. Irish (even though
ag so, ag seo, etc. were still written in literary works well into the 18th, even 19th c.). Breatnach suggests it’s then that the construction was fully identified with the copula in meaning: the copular sentence of identification like
as é so sgiúrsa as géire do léig Dia chugainn ‘this is the harshest scourge that God let upon us’ and
so an sgiúrsa do bhagair Dia ag Esaias ‘this is the scourge that God threatened through Isaiah(?)’ in
Scáthán Shacramuinte na hAithridhe – one with full copula expressed (
as é so…), the other with copula omitted (
so…) – led to identification of the second form with the
ag so… expression. This led to classification sentences like
seo madra ‘this is a dog’ (from
ag seo madra) even though no such copular form as
*is é seo madra exists¹ (and the VPS order,
(is) madra (é) seo, is required) and generally one doesn’t say things like
*capall ainmhí for ‘a horse is an animal’ (though Breatnach notes the similarity to nominal clauses like
tosach maith leath na hoibre and
bean ar meisce bean in aisce with the predicate and the subject in the “wrong” order, I’d add
tír gan teanga tír gan anam to the list).
Then, during 18th–19th century, other forms of the copula started to be added to the (
ag-less) phrase:
rádhais … gurab súd an bhean; gurab sin an tslí. Also grammars of the time tried to explain the construction as elision of the copula. Peadar Ua Laoghaire in his early texts wrote things like
is seo focal le Diarmuid showing this kind of reanalysis of those phrases as containing the copula.
And at that time – by analogy with copular forms with the pronouns:
’sé, ’sí, ’siad – these
seo, sin, siúd were reinterpreted as
’s eo, ’s in, ’s iúd – ie. the copula + elements
eo, in, iúd – which led to the substitution of the initial /ʃ/ by other copular forms, like
b’in í an cheist; nach eo é do mhac? in Munster and Connacht (while in Ulster generally the copula is just added to
seo, sin, siúd:
ba seo…, ba sin…).
Another article,
Some Notes on the Demonstrative-Initial Construction by William J. Mahon (1984) in
Proceedings of the Harvard Celtic Colloquium, vol. 4, follows modern (after the loss of
ag) developments and gives many more examples of 19th and 20th century usage, including some examples of
seo/sin é in classification clauses before an indefinite predicate (
sin é casíno mór) – Mahon also gives
sin é cleas bhiodh aguinn ’nár ndúthaig, but IMO that’s definite by the virtue of being defined with a relative clause, so I’d say he made a mistake describing it as indefinite.
The article also shows the resistance in Ulster to segment the demonstrative as copula
’s +
eo, in and to treat it as copular identification clauses (eg. shows
’sea as a response in
“Seo an áit a bhfuil tusa?” ar sise leis. “Sea,” ar seisean in Donegal – explained as agreeing with the whole proposition, instead of the ‘expected’
’s é mirroring the perceived copula).
Mahon also shows early 19th c. Munster and Connacht identification examples without the pronoun:
Siud na diabhail (…); Sin an uair. So an lá.; Nach so an focal (…), etc. Showing that the use of the pronoun before definite (but rarely indefinite too) predicates is a 19th century development.
¹ at least in Ireland, I believe in Scottish Gaelic the reanalysis went a bit further and
is e seo cù does exist in Sc. Gaelic