Is this correct?
Seo/sin iad/é - here/there they are/it is | this/that is them/is it
Seo/sin iad would perhaps be better rendered as “these/those are them”.
You could also add siúd for a further contrast — seo/sin/siúd for “here/there/yonder” and “this/that/yonder”.
"taispeáin do leabhair dom" - "Seo iad"
"Show me your books" - "Here they are". ("These are them" sounds a bit off to me)
Seo iad na leabhair - "Here are the books" or "these are the books" both work - you might prefer one over the other depending on context, so "An iad seo na leabhair?" would be "Are these the books?".
I agree; I was contrasting “these/those are them” vs. “this/that is them”, not vs. “here/there they are”.
You've got a complicated matrix there, but , as danryan0 points out, they all work.
Really? I've been learning for quite a while and this is the first time I've heard of it. Is it more common in all parts of Ireland?
In both Munster and Connacht dialects yeah.
It's really common in spoken Irish and works written by native speakers, but for some reason most learning material doesn't present it.
The Caighdeán's "Seo é" is based on that being the form in Clare Irish, on which the Caighdeán is mostly based (unfortunately now extinct).