Is an irregular heart beat usually atrial fibrillation?

Not always: It certainly can be. Although there are multiple instances when one can have an "irregular heartbeat'. You should see your doctor so an ekg can be done or possibly a monitoring device ordered to evaluate further.

Usually: Atrial fibrillation is certainly one of the most common causes of an irregular heart beat, but not the only one. Probably most common are benign premature beats which many people have and are of no significance. For a potentially dangerous irregular rhythm, atrial fibrillation is the most common.