You need to read the Revenue Leaflet SD10A - available on their site. That sets out the rates etc.,etc. Look at the certificates that go into the Deeds and specifically Certificates 8a and 8b.
Once contents are included and overall price (including contents ) pushes price into a higher stamp duty bracket the rule is stamp duty is payable at the higher rate on the lower amount. Lots of people will tell you that there are loads of ways to get around this - separate contracts, cash deal etc.,etc - all are fraudulent, and if you that route, you need to involve the solicitors and/or auctioneers and they will not ( and should not ) get involved.
mf