The first one is tricky. The other three are well-known
First of all units can only be from polynomials that are constants.
For irreducible polynomials over $\mathbf Q$ one has well-known Eisenstein theorem that guarantees one in each degree (alternately simpler result about the integers: like $\sqrt[n]p$ is irrational for any prime number shows that the polynomial $X^n-p$ is irreducible for every $n$.)
For complex numbers we use Liouville's theorem and conclude that all polynomials have roots, allowing us to conclude that irreducible ones have to be of degree 1. From this one can conclude that irreducible real polynomials have to be either in degree 1 or 2, and not higher.
Primes and irreducibles are the same in them is a consequence of the fact that this is a PID.