This question came up last year as well. The organisers of the Boom festival, while saying they want to protect the environment, have not chosen a location exactly easily accessible by Public Transport!!!!!!!!!!
Idanha-a-Nova is 262kms. from Lisbon, about a 3 hour drive. It is located in a rural area close to the Spanish Border.
There will certainly not be a public bus service at that time of night.
I really would recommend getting the Bus. (i.e. fly in perhaps the day before and spend a day in Lisbon)
One could catch a train from Lisbon Gare Oriente to Castelo Branco, the city nearest to Idanha-a-Nova. However, the summer train timetable will not be out till late April. The current train timetable shows the latest departure at 19h26 arriving 22h09. She would then need to get a taxi (if avaliable) from Castelo Branco to Idanha-a-Nova. (Distance +-40 kms.)
There is also a bus service operated by Rede Expressos to Castelo Branco but the last departure is at 19:00 arriving 22:35. Again a taxi would be needed to get to Idanha-a-Nova.
