I know the general rule that, when conjugating verbs in the present, the amount of vowels in the root verb determines whether it is first or second conjugation. I also know that there are certain irregulars ending in "igh" (for example, "nigh") which are first conjugation (one vowel) but operate as second ("ním, níonn") when conjugated.

However, I'm very confused by verbs like "taispeáin," "scríobh," and "caith," which despite having two or more vowels, are treated like first conjugation verbs (i.e. "scríobhann" instead of "scríobhaíonn"). Which conjugation are they technically in? Is there any rule or pattern to help me remember which verbs with multiple vowels act like first conjugation?

Go raibh maith agaibh mo chaire.

That’s an oversimplifying rule.



No, níonn is just a simplified spelling for nigheann (resp. ním / nighim)
As well as ceanaíonn is just a simplified spelling for ceannuigheann (the way it was spelled until 50 years ago)
The real difference between first and second conjugation isn’t -íonn or -ann but the future tense forms -fidh (1st) and -óidh (2nd).
Nigh in future tense is nífidh and not "neoidh", "níóidh" or something like that, so it is first conjugation.



This "general rule" above could only hold true if you'd consider only the spoken vowels.
Scríobh /s´k´r´i:v/ or caith /kah/ don’t have more than one spoken vowel, so first conj.

Taispeáin and most verbs with a long vowel in its second syllable are first conjugation.

And, of course, there are exceptions.

It's the number of syllables, not the number of vowels (and, as Labhras pointed out, it's a very general rule of thumb. There will always be exceptions. When in doubt, always consult a dictionary.

"Scríobh" and "caith" are single-syllable verbs, and both are first-conjugation.

Redwolf