You have to get on to the C31 and follow it until you're nearly in Barcelona, turn off at 1st sign for Fira. After that it's easy peasy, you can see the Ikea building and the traffic system seems sorted now. BTW,there isn't an IKEA at the Tarragona Centre Commercial.

PS. The large IKEA sign that was on the motorway has been removed, so you gotta watch for that Fira sign.