Here is Ó Siadhail’s description in Learning Irish:
“There are two basic types of regular verb; in the vocabularies, verbs are marked ‘1’ or ‘2’ according to whether they belong to the first or second type. The second singular of the first type has one syllable, e.g glan, bris, nigh, léigh while that of the second type has two syllables, e.g. salaigh, coinnigh, oscail, imir… Exceptionally, verbs with roots ending in -(e)áil (pacáil, péinteáil etc.) belong to type 1; also verbs spelt with one syllable but pronounced with a helping vowel, e.g. dearg ‘redden’ belong to type 1.”