The foundation of many phobias is the development of a classically conditioned fear response, which then fails to disappear because the person avoids the feared stimulus. However, not all phobias are acquired through classical conditioning. Many individuals who experience phobias are unable to remember any particular event before they started experiencing their phobia. Additionally, it's surprising and interesting to note that most people are fairly resilient when they experience frightening events and do not develop chronic fears as in the case of phobias as well as in PTSD. For example, the vast majority of people exposed to bombings by airplanes during World War II endured them pretty well, having short-lasting fears that were very rationale considering their situation, and those fears dissipated quickly thereafter. Researchers in the field of psychology have proposed that additional variables likely play a role in obtaining phobias as well. Some of these variables include; observational learning, temperament, preparedness, incubation, US revaluation, and selective sensitization.
In my own personal experience, I have had what I would categorize as a phobia of heights (acrophobia) for as long as I can remember. I am not quite sure where this fear came from as there is no event that I can recall but I know that any time I am in an elevated setting I get filled with a deep sense of anxiety.
As someone of Irish descent myself, I have traveled to Ireland a few times and have also gone to the Cliffs of Moher once or twice. I can remember that I couldn't muster the courage to go within even 100 yards of the edge and seeing people that were brave enough to look over the edge made me feel anxious for them. I found a very entertaining video of a guy who also has a fear of heights and tried to face his fears by slowly approaching the edge of the Cliffs of Moher. Hopefully, I will be able to conquer my own phobia and be able to do this one day!