After the noun, yes, but not necessarily before the adjective.
The adjective here is the predicate, separate from the subject. Tha (an soitheach seo) (àlainn), ‘(this dish) is (lovely)’. That’s why it is after seo (after the whole subject, an soitheach seo).
If you wanted to say ‘this lovely dish is big’, you’d say for example tha an soitheach àlainn seo mòr – here an soitheach àlainn seo is the subject (notice that seo goes after the whole noun phrase) while only the predicate mòr goes at the end.
Great explanation ... I would expect any pause in this example phrase to be between "seo" and "àlainn" - "an soitheach seo" being the subject of the sentence (some languages will use slight pause for semantic emphasis), whereas a phrase like "Tha an soitheach cho àlainn" would have a different rhythm. Is pause and rhythm prevalent in this way in Gaelic?