In the film “Barry Lyndon” the narrator states that the Prussian military had native Prussian officers but most of the grunts were made up of criminals, drifters, mercenaries and scoundrels from every nation in Europe. The main character of the film, Redmond Barry, was an Irishman serving in the British army who was forced into the Prussian army when they caught him deserting. This goes against my pre existing idea of what the Prussian military was like, I was under the impression that it was ethnically homogenous with everyone speaking German. Is this true or was it made up for the movie?
Your impression is vastly incorrect I'm afraid. This is why they caution us about making assumptions.
The army of Frederick (II) the Great during the time in question, the Seven Year's War, was indeed filled with foreigners of all kinds. Many of them made up of the rougher sort that Prussian recruiters had managed to cajole, bribe or trick into service. They would pay bounties to get recruits, which didn't always turn out so well.
From one third to half the army was foreign during his reign. And during peace-time when Prussian citizens, albeit in the army, could return to their civilian life for most of the year (excepting 2 exercises months) the army was majority foreign. Note foreign here also includes other German speakers.
One major reason of the personnel turnover was endemic desertion. Not surprising when recruiting everyone willing to take the money. Fredrik II himself noted his army contained half such chancers. This led to draconian punishments and measure to combat desertion which ofc would also encourage people to leave the harsh regiment. But there were also plenty of wars that swallowed regiments whole.
The officer corps similarly contained foreigners, the main thing Fredrik II cared about was competence and that they were nobility, even if just created so to be able to be officers. In these matters Fredrik didn't really care overmuch about your religion or other such things. To some degree oddly egalitarian in the vein of the enlightened despots of the time.
Can read more about Frederik the Great's army in the 3 volumes of Osprey's MAA series no 236, 240 & 248 "Frederik the Great's Army" volumes (1) Cavalry, (2) Infantry and (3) Specialist troops respectively, are interesting and somewhat lighter reads.
So yes Barry Lyndon is afaik pretty accurate in that regard, even the king himself didn't have an entirely high opinion of some of his troops. The wars and desire for a powerful army just meant you took what was available.