Is ερωτήσεις an irregular plural?
Not irregular, some nouns in -η form plural with -ες and some others with -εις. there is really no way to know beforehand, you just have to learn them by heart.
The ones in -εις usually end in -ση (-ψη -ξη), which is a hint.
(But not always: e.g. δυνάμεις, πόλεις.)
That's true, but there are also exceptions the other way around: βρύση-βρύσες. But most of the times what you say is true. ;)