During one of the battles, the Irishmen says to Wallace "The Lord says he can get me out of this, but he's pretty sure, your f*cked!" This word was actually an acronym meaning "Fornication Upon Consent of King" prior to it becoming a vulgarity many centuries later. So why would the Irishman use it as such during this time period?
Your belief in the origins of the word aforementioned is incorrect. The "fornication under..." is actually a clever acronym some one made up, no doubt to impress chicks at parties. It actually stems from the German "fekkher," meaning to prick or to puncture. A considerable amount of English is Germanic-based, and no doubt the word "f*ck" existed around the time of this movie's setting. However, I don't know whether or not it was a vulgarity at the time, or if it was as prominent as it is now.
Actually, the F word is derived from the Anglo-Saxon verb "Fokken" which meant "to strike" or to bang against. When the "sophisticated" Normans invaded England in 1066 they established most of the Anglo-Saxon language to be vulgar in comparison to their French dialect. Thus the Old English word for pooping (Sh*t) became a swear word and the term "Defecation" was considered proper. Fokken was among the words declared as vulgar in the 11th century. It would have been perfectly plausible for an early 14th century Irishman to curse in such a fashion.
Sorry, but you may both be wrong. If one looks up the word f*ck in the dictionary, and then follows up with a little research, you'll find that the word actually means "For Unlawful Carnal Knowledge".
Since an Irishman (or Scotsman for that matter) in the 1300's wouldn't have been speaking English to begin with (they spoke Gaelic almost exclusively up through about the mid-1800's and later), then this whole complaint/discussion is really a moot point.