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

That’s true. It happened in French too: être étais are from Latin stare, and est sont serai are from Latin esse. In fact it happened in Latin as well; some forms of esse (fui futurus, etc) are built on a different verb.

An example of when this isn’t the case is any Germanic strong verb – an “irregular” verb that has a vowel change, like sing sang sung, find found, drive drove driven etc. Many of these can be traced back to regular vowel alternations in Proto-Indo-European. Others, like catch caught, think thought, were weak (regular) verbs that became strong.

OP: You might want to take a look at Steven Pinker’s Words and Rules for an accessible discussion of irregularity. His theory, IIRC, is that vocabulary and syntactic rules are stored in different parts of the brain. We store all forms of the most commonly used verbs as vocabulary, while for the less commonly used ones we only store a base form and generate the rest from rules.

The past tense of go is an example of what you are talking about, being derived from the archaic verb wend.

Of course, some irregularities are holdovers from when English verbs were inflected for person. Some non-standard dialects dispense with these remnants.

Yes, that is usually how irregular verbs come to be (and irregular verbs are often the most “basic” verbs as well). Interesting question. I’d like to know if there are any examples of irregular verbs with complete stem changes (like the Latin ferre, tuli, etc.) that did not come from different verbs—seems unlikely.

@morphail Do you know what the “fui/futurus” forms derive from? I’ve always wondered that…

Interesting topic on which I know little. I always wondered why the present tense conjugation of the verb to be consisted of is and are.

In all languages, irregular verbs tend to be the most common. It is not difficult to see why. Humans are wired for creating language. The rules for a particular language come after the fact. Initially, there are no rules for conjugation. Several different patterns develop. Eventually, the conjugation of a particular type of verb becomes the norm for regular verbs.

@submariner What do you mean by “some irregularities are holdovers from when English verbs were inflected for person”?

@DominicX fui and futurus are from the Proto-Indo-European bʰeuH- “be, grow”. This is also the source of English be.

^E.g., Irregular 3rd person singular forms like does and has. Of course, even the standard regular 3rd person singular forms are holdovers in this sense. Non-standard dialects have continued the trend and dropped the inflection for person altogether: I says, you says, he says, etc., or I say, you say, he say, etc.