The campus of Instituto Superior Tecnico (IST) is now easily reachable by subway from the airport (it's a 16 minutes ride): take the red subway line and exit at Saldanha to Av. Duque de Ávila or exit at Alameda and go up the hill (zoom out and click on the three red markers on the map).
Donwtown Lisbon is also easily reachable by subway from either of these two stations. Exit at either Rossio or Baixa/Chiado.
Venue: amphitheatre of Complexo Interdisciplinar (click on the red marker in the middle of the map).
We have made a block reservation at two of the hotels listed below, which are all near the subway station Alameda (when booking, make sure to mention Iberian Strings 2017 - all rates per day and including breakfast):
You may also want (or may have) to look into other options/locations: this should not constitute a problem.
Lunch may be taken on the premises, or at the restaurants surrounding the campus.