By historical review it would seem that:

1) Irregular warfare is the norm
2) Regular warfare is the exception
3) Irregular warfare kills with a thousand small cuts (queitly like cancer)
4) Regular warfare kills far quicker and dramatically (loud like a car crash or sudden like a heart attack)

Regular warfare tends to get the most attention becuase it is dramatic, it is loud, the results tend to be decisive and it is generally short 4-6 years.

Irregular warfare tends to be forgotten about, pushed to the back pages of the newspapers, footnotes on the news. Generally a few people killed here and there brief flare-ups on occasion, hard to keep people interested. Last 10, 20 or more years. The level of conflict becomes accepted, the death toll becomes the price of doing business like the annual US Highway Deaths.

Basically irregular warfare exists to some extent all the time and when a regular war breaks out the two blend together. This is when difficulty arises-when the two blend to create, to steal the current phrase, a 'hybrid' war.

My best assesment is to view irregular warfare as a style of war similiar to the idea that Judo is a style of martial arts. As in Mixed Martial Arts you can fight with any style you want, but you can only fight with the style you know. If a Judo practitioner gets himself into a street fight, no holds barred, guns, knives, kitchen sink kind of fight he will use everything and anything. He'll just have to adapt and overcome to survive.

Regular warfare is also a type of war, and the same concept applys but becuase it is seen as so destructive and so costly it is most often avoided and can generally only be fought by Nation States because of the costs involved.