There is some textbook literature which essentially covers the issues raised here, though it often deals with more general situations.   (Over finite fields life is simpler, since Schur indices are 1.)   See for example Curtis & Reiner (1962), Section 70, and also the book *Character Theory of Finite Groups* by Isaacs (reprinted by AMS), especially Chapter 9 and problem (9.6).   

The basic criterion deals with an arbitrary finite group $G$ and its representations over a finite splitting field $\mathbb{F}_q$.  In your situation, this applies to finite Chevalley groups by the 1963 paper of Steinberg (see $\S13$ of his 1967-68 Yale lectures <a href="http://www.math.ucla.edu/~rst/">here</a>).  What you need to know is that *an absolutely irreducible $FG$-module $M$ over a subfield $F$ of the splitting field $\mathbb{F}_q$ is irreducible over the prime field $\mathbb{F}_p$ if and only if $F$ is the "field of definition" of $M$*.  Here a field of definition is one generated over the prime field by the traces of representing matrices.  (It's probably hard to find an explicit statement and proof of this in the textbooks, but it's implicit.)   

In your specific example, it's therefore necessary to be careful about where the traces of the representing matrices lie.