Probably figured it out myself, so if anyone's interested, here is my solution:
$(\gamma^5)^2 = 1 $
$ \gamma^\mu \gamma^5 = - \gamma^5 \gamma^\mu$
$\Rightarrow \gamma^\mu e^{i \Lambda \gamma^5} = \gamma^\mu \sum \frac{(i \Lambda \gamma^5)^n}{n!} = (\sum_{even} \frac{(i\Lambda)^n}{n!})\gamma^\mu+ \gamma^\mu(\sum_{odd}\frac{(i\Lambda)^n\gamma^5}{n!})$
$= (\sum_{even} \frac{(i\Lambda)^n}{n!})\gamma^\mu - (\sum_{odd}\frac{(i\Lambda)^n\gamma^5}{n!})\gamma^\mu$
$= (\sum_{even} \frac{(-i\Lambda)^n}{n!})\gamma^\mu + (\sum_{odd}\frac{(-i\Lambda)^n\gamma^5}{n!})\gamma^\mu$
$= e^{-i\Lambda \gamma^5}\gamma^\mu $
Massless Dirac Eqn.:
$i \hbar \gamma^\mu \partial_\mu \Psi = 0$
$\Rightarrow i \hbar \gamma^\mu \partial_\mu e^{i\Lambda \gamma^5}\Psi
= i \hbar e^{-i\Lambda \gamma^5} \gamma^\mu \partial_\mu \Psi = 0$