There are a lot of French verbs that end in -ER and there are a lot of irregular French verbs, but there is only one irregular -ER verb. However, there are three groups of -ER verbs that have some irregularities.
One True Irregular -ER Verb
Aller (to go) is the only truly irregular -er verb in French - its conjugations are unique and, according to some, very odd.
Spelling Change Verbs
Spelling change verbs are verbs that end in -cer or -ger. Their stem formation and verb endings are the same as for regular -er verbs, but there is a slight spelling change for pronunciation purposes in certain conjugations.
Stem-changing verbs are -er verbs that take the regular endings but have two different radicals. There are five categories of French stem-changing verbs: -yer, -eler, -eter, -e_er, and -é_er.
There is nothing actually irregular about the conjugation of -ier verbs - they are conjugated like regular -er verbs, but some of their forms look strange.