Normal adult resting HR is 60-100 but lower than this can be normal too.
Very fit people and often teenagers tend to have a lower heart rate
because they have better cardiovascular efficiency; in this instance a very low HR can be a completely normal healthy indication. My resting HR is often below 50.
The fitter you are, the larger stroke
volume one has and this means more oxygen
is distributed to the body with each heart beat. So the fitter you are, the less your heart has to pump to do the job. It's the exact same when you rest and the exact same applies when you exercise.
A very low pulse rate can be tolerated as long as the amount of blood pumped out of the left side of the heart per minute is adequate to oxygenate the brain and the other parts of the body. Once the heart is unable to fulfil this requirement you will become symptomatic.The level this is tolerated at varies from person to person. Below 40 is usually always a problem and is absolute bradycardia
As I said before, your HR is within normal range so there is no real cause for concern. If you are experiencing other symptoms and/or this is a new development (you mentioned irregularity), it's worth seeing your GP ro rule out other underlying problems. Also, certain medications can cause low HR. What is considered "normal" really depends on the person and to some extent, situation.