"This is a chicken."
I think it's classed as a copula.
It's basically both a verb and an adjective, in the same way that "am" is in English.
Seo, sin, and siud, are actually shorthand forms. The full, older, version is 'S e seo.
Seo means "this", but the 's e part isn't really used and is assumed. So "seo" is taken to mean "this is". Because it's not technically "seo" which is the identifying particle, "seo" can be flexible where it goes in sentences.
Dè tha SEO? - What is this? SEO uinneag - This is a window