I doubt very much that they'd complicate their slick operational model by introducing the shorter model to revenue service.
But for the auxiliary role it was cheap to purchase or lease versus the -800. This particular airframe was stored at Shannon for nearly a year before being picked-up by Ryanair.
Also rumoured to be fitted with a couple of rows of business-type seats for the corporate shuttle role, might as well fly the boss around in a liveried aircraft instead of the pair of Lears.