By its very definition, terrorism is designed to terrorize and harm the general population, including wholly innocent men, women and children. No matter how important the perceived issue motivating a person or group to resort to terror, engaging in terrorism and harming or even killing innocent men, women and children cannot be justified.
There is no justification for terrorism, even if it used fighting for a just cause. Some people believe the ends justify the means, but has terrorism ever been effective? The bombing of Nagasaki and Hiroshima helped end a war, but the actions still cause harm to Japanese and brings shame to Americans. Terrorism was used to fight for and against civil rights, but it was the non-violent demonstrations that really made the difference in the movement. No matter how you use it, terrorism is immoral and an ineffective way of getting people on your side.
No, there is never a situation in which terrorism is justifiable. Terrorists are at the ends of their wit or are religious, but there should always be room for conversation and debate. Violence is never the answer to problems because it only creates more problems. Instead of blowing up cars in crowded neighborhoods, terrorists should reflect about the harm they cause for no reason at all. The chaos is avoidable with simple words.
Terrorism is a way of expressing disagreement through violence with the intent to instill fear. If you must use fear to force someone to accept your beliefs, then you have not really achieved anything because people are only pretending to believe as you do for fear of what may happen if they don't. This is tyranny, and it is never acceptable.