Did most irregular verbs originate from the merging of multiple other verbs?

I remember reading somewhere that the modern English verb “to be” actually originated from many different old Germanic verbs whose usage merged into one verb. Eventually, these multiple verbs effectively became, and then actually became, the same verb. The irregularity of the new verb is because of the fact that the different forms of the verb come from different original verbs.

So, is this the way that most irregular verbs were created? How common is it for a language’s verbs to merge? What are some examples of when this isn’t the case?