I was working on the demonstratives and articles for my Vindonian language one day. I eventually shifted to the personal pronouns and the possessives. As I thought about these, I decided it might be useful to make my own reconstructions of the Proto-Indo-European 1st and 2nd person plural pronouns and the plural demonstratives (i.e. 3rd person plural pronouns). These are given in the following table along with their Proto-Celtic reflexes:
PIE pronouns PC pronouns
nom acc nom acc
1pl incl hnes hnṇs (hṇs) snīs ans
1pl excl wejes wejṇs — —
2pl juhes juhṇs swīs was
3pl hejes hejṇs ijes ijas
(Of course, the h’s in the PIE forms stand for laryngeals, but without the usual subscript numerals. The 1pl incl forms are usually not reconstructed with an initial laryngeal, but I think the accusative form should require it. The laryngeal in the 2pl forms are apparently undefined. But the larygneal in the 3pl forms was most probably h1.)
Perhaps I should explain the changes between PIE and PC in nominative forms of the 1st and 2nd persons:
1pl hnes > nes > nēs > nīs > snīs
2pl juhes > (i)wes > wēs > wīs > swīs
The lengthening of the vowels is paralleled in Latin (nōs, vōs). By the way, the addition of the initial s in both forms is unique to the Celtic languages.
Although the accusative forms of the PC pronouns aren’t found in Celtoid, the Old Irish possessives ar ‘our’ and for ‘your’ come from *ans-ros and *was-ros which are derived from the accusative forms of the personal pronouns (as also in Latin and in Germanic).
I should also point out that I believe that there was originally clusivity in the 1st person plural pronouns in PIE but that this was eventually lost somewhere along the way. This inclusive/exclusive distinction in 1st person plural pronouns is found in many languages in several language groups. Clusivity in PIE would obviously account for the two morphologically irreconcilable 1st person plural forms shown in the table above. I might speculate that the exclusive form (*wejes) is related to the *wi– prefix which denoted separation/apartness in PIE.