There have been ongoing discussions about the possibility of allowing low-cost operators to use one of the airbases around Lisbon, just like Ryanair.com and Wizzair.com use Ciampino Air base outside Roma.
The airbases that have been examined include Montijo, one the south side of the Tejo, just the other side of the Vasco da Gama bridge, Alverca about 20km North of the current airport at Portela and Sintra air base. The study has been going on for the past two years and although decisions are constantly promised in "the near future", to date nothing has been announced.
I can see why!
Public transport options from Sintra to Lisbon is limited and the IC19 motor-road between the two is close to saturation.
Alverca has a technical problem in so far as its approach and take-off paths may clash with those from Portelas Lisbon Airport. It would be the easiest and most convenient as there is a station on the Lisbon-Porto line just near the airbase.
Montijo is the most often talked about option as it would also benefit tourism developments near Setubal and elsewhere in the Alentejo. However, public transport into Lisbon could be a problem.
If you have a smattering of Portuguese, follow Sergío Bastos excellent website
Sergío is usually the first to publish news on new developments in the Portuguese commercial aviation world! (except on the odd occasion when I have beat him to it! ;).
If anything is announced about this, he will be one of the first to know.
However, personally, I think it is at present a long-shot given Portugal's current ongoing economic and political shenigans!
A final interesting point is that the IATA winter 2013/2014 slot requests for Lisbon Airport were released last week and Paddyair.com, (who at present do not operate from Lisbon Airport) have requested a substantial (read a stupidly high) number of slots. There has been no announcement from St. Michael of O'Leary about possible Lisbon routes but this may be start of negotiations with the new operators of Lisbon Airport, to figure out a way to make it commercially possible to enhance low-cost flight operations from Lisbon. (I suspect an agreement on landing and handling fees will be the main stumbling block.)
See (in Portuguese): http://bit.ly/12KZeoj