Why is are weak acid-strong base and strong acid-weak base reactions irreversible?
Yes the following reaction is reversible. 

$$\ce{CH3COOH + OH-  &lt;=&gt;  CH3COO-  + H2O}$$

There is an equilibrium between the species $\ce{CH3COOH, OH-}$ and $\ce{CH3COO-}$ in aqueous solution. 

Thus the gist is that particular $\ce{CH3COOH}$ and $\ce{CH3COO^-}$ molecules aren't stable, but rather continuously react.