Editing Openai/69186d20-2d14-8013-b9c6-c09000f173f6 (section)

=== Assistant: 2511141554ET ===
251114_1554ET

short answer: siad “they” < Middle Irish íat < Old Irish íat (3rd plural pronoun), ultimately from Proto-Celtic *ɸīdat or similar; it’s a very old inherited Celtic plural, not a recent formation.

===== - siad /ʃiəd̪ˠ/ - 3rd person plural subject pronoun: “they”. - Usually used after the verb in normal VSO order: - Labhraíonn siad Gaeilge. – “They speak Irish.” =====

There’s also iad (object / emphatic variant) and emphatic siadsan.

===== Wiktionary gives: =====
'' siad < Middle Irish ''íat*.

Old Irish has 3pl pronouns like:
* íat (independent / emphatic) “they (pl.)” – usually post-copular / emphatic.

Over time:
* Old Irish íat > Middle Irish íat (same spelling, evolving phonology).
* Consonantisation / reanalysis in early Modern Irish created siad with an onset [ʃ] (analogous to sé, sí), while iad survives as the “bare” vowel-initial form.

So synchronically:

: 

===== Reconstructing plural 3rd-person pronouns is messy, but the consensus is that Old Irish íat continues: =====
'' a Proto-Celtic 3pl demonstrative/pronoun, sometimes reconstructed as something like ''īdat / *ijōs, from PIE deictic elements (same broad family that gives Latin illī, eī, etc.).

The important bit for your mental map:
* sé / sí and siad / iad are all old inherited Celtic pronominal forms, reshaped analogically in Old/Middle/Modern Irish.
* The s- of siad is not original; it’s a later leveling to make the 3pl subject pronoun look like the 3sg ones (sé, sí).