The supplier's TRN must appear on every tax invoice, and the customer's TRN too where the customer is registered. A missing or wrong TRN can cost the customer their input VAT recovery.
TRNs can be checked on the FTA's verification service. Under the eInvoicing programme they also identify the parties in each structured invoice.
How Civova handles it
Civova holds the TRN of your company, each customer and each supplier, and prints them on every tax invoice and credit note.