I have a form to send to the UK which a Freepost address was given (no printed envelope supplied). Is it still free if sent from the Republic of Ireland?
I would imagine not. The cost of mail to Freepost addresses is paid by the recipient (usually a business, or a Govt. department) who would be billed by An Post in Ireland, and Royal Mail in the UK.
AnPost's website doesn't offer any relevant info in their FAQ section, and their search function doesn't seem to work — but I don't think there's any reciprocal agreement in place that would allow for what you're describing.