When I was tracking my packet from Ellis Clark with Royal Mail, there was a link to continue tracking which automatically brought me to the An Post track and trace page. The packet number was there with the An Post status.
When the packet was ready for dispatch, Ellis Clark sent me an email with the tracking number. Clicking on this number automatically linked to the packet status on the Royal Mail tracking system.
As I paid the VAT to An Post today, it will be interesting to see when it arrives with me.