Some thoughts about "defective" verbs and other fossils.
Word has it that "sene" became a fixed form, attached to "andar" and that the verb *"andarse" (or *"andarsi") never existed. Or did it.
So there is no
Me ne vado
And this also works:
Me ne vado a festa
(Cf French : Je m'en vais)
*Je me vais
What happened in Spanish ?
Me voy (Spanish has no clitic pronouns of this type - genitives or locatives)
Andate! Vete! Vayanse work (in some varieties of Spanish)
So "andarse" in Spanish does not exist as a complete form (in certain dialects) and certainly there was never an "andarsene" in Spanish (but ! hay has a fossilised "y" in "ha-y"). So there must be some locatives out there that got fossilised into the verb. And other forms that are happy to become reflexives in part.
Latin: to carry is fero ferre, tuli, latus
Latus sum = I am carried (perfect passive of ferro)
Why did fero "steal" the perfect forms of tollo (if it didn't have one, it could have been invented).
Saying that forms are irregular explains nothing. If there is "andarsene" there must have been "andarse", if there is "andate" there is "me ando". At some stage.