It all depends on the mechanism behind the irregular heartbeat. Typically irregular heartbeats are generated in the atrium, which are the upper two chambers of the heart, there is a nonconductive layer of tissue between the upper chambers and the lower chambers called ventricles, in between is a failsafe gate that delays signals between the upper and lower heart preventing runawy ventricular beats. IN GENERAL ventricular irregularities are bad. Fortunately they are a small minority of what we see. For perspective every cell in the heart can generate a heart beat, that is why everyone will get palpitations every single day. Sometimes the atrium will just go berserk and just fires off signals, these signals are prevented from reaching the ventricles by the failsafe delay called the av node. That is the most common irregularity called atrial fibrillation. It feels like a bad drummer doing a solo in your chest. That can be caused by several factors, basically anything that irritates the cells of the heart, like alcohol, caffeine, ephedrine, excess thyroid, certain medications, structural abnormalities, or just because, which was my case and how Im in my profession, just read so much on it decided to go to school for it. The danger of irregular atrial rhythms is more a function of the result, if your heart is prevented from producing enough output because your heart is beating to fast to fill the chamber for every beat, low bp and cardiac output will result with symptoms accordingly, Also if you stay in irregular rhythm like atrial fibrillation, blood can coagulate in the atrium then shoot up to the brain and cause a stroke. But, thats worse case scenario and is unlikely because people typically go to the doctor if they have a rhythm that lasts a long time (over 48 Hrs).
This is meant as educational and should not be used in place of appropriate medical care, there are dozens of different heart rhythms caused by humdreds of different things and this is meant as a basic overview, I will address specific questions if you post, but in an educational environment. Later for now.