There is are several buses daily from Coimbra (Portagem) to Praia da Mira.
I would recommend:
1) Arrive Lisbon Airport
2) Immediately get the Metro or taxi (Watch the latter! We had yet another contretemps about fares last night with the driver we who took us to Lisbon Gare Oriente Station to get the RENEX bus to the Algarve. We were running late and had no time to take the Metro. Luckily we both speak Portuguese and the driver lost!)
3) Get the 21h27 train from Oriente to Coimbra-B (repeat Coimbra-B) arriving at 23h36. (www.cp.pt)
4) Take a taxi (Safer at that time of night) to a hotel near the Coimbra Portagem area.
5) Have a look at the Residencial Vitoria (my favorite), the Hotel Ibis or the Residencial Larbelo. I have stayed in the first two. (Tell them you are arriving v.late)
6) In the morning, ask for directions to Portagem (Along the river front, a 100m from each of these Hotels.)
7) Transdev/Joalto Mondego have departures at 7:15, 10:00 and 13.05 all of which go to Praia da Mira. There is also an 08:05 that terminates in Mira.
Transite time is 1h:30mins.
Timetable here (pdf download): http://goo.gl/Tqpcb
