memorize or logic to irregular verbs
Is there a logical method to know which verbs are irregular verbs, or do we need to memorize all of them without rhyme or reason?
I'm really torn over anwering questions like this. As mentioned, there are a very tiny number of truly irregular verbs in Spanish (ser is one of them). The rest are irregular in a regular way - tener, poner, etc. So you don't have to use brute force memorization for a whole herd of verb conjugations. You just need to know the patterns and which pattern to use with which verb.
On the other hand, if you have to think about it, you'll never be able to hold a real-time conversation. That requires internalizing, not memorizing. Even if you can perfectly picture the conjugation table for every single verb, that won't be fast enough. The words have to go in your ears and create meaning in your head, and the meaning in your head has to flow out of your mouth as words, without your brain getting in the way. Conjugating English verbs is much harder than Spanish, and you do it without thinking about it. The only way to do that is through talking, listening, reading and writing, a lot. While we all have to put the time into drills and memorization, reading a newspaper, watching TV, or chatting with friends in the target language is much more useful.
Irregular verbs have patterns to them, which you will begin to recognize over time.
Here's an article on How to Learn Those Pesky Irregular Verbs.
There are degrees of 'irregularity'. A large class of verbs are what are often call "stem-changing" / "boot" verbs Their irregularities consist mainly of systematic substitutions of a vowel for a /vowel/diphthong (or vice versa) so one needs only to learn certain patterns. Other verbs are "all over the map" and must be memorized (unfortunately, in most languages, these are among the most commonly used verbs).
There is a logic, irregular verbs do go into groups which conjugate the same way. I had a list of some of the groups, but can't find it now sorry. :(