Talk to the auctioneer for an idea as to what market you should be aiming for. If it's investors it can be marketed with sitting tenants. But if it's for owner occupiers presume better not to mention tenants, and might be necessary either to wait until end tenancy or give notice to tenants.
Personally I wouldn't use an auctioneer but advertise on one of the websites - my home.ie etc. (I've no interest in the company). Either that or agree a fee with the auctioneer to include advertising etc. But if you are in no hurry try to sell it yourself.