Beircheart (bar-hart, bear-hart, and bear-kart) is an Old English and Irish baby name most people have forgotten about. It looks a bit like bear + heart or birch + heart, especially when you say it out loud phonetically (BEAR-chart). It means "[of the] intelligent army; bright army" and has roots in Anglo-Saxon, also commonly used in Irish. In a few sources, a Latin name is given: Berichertus.
Benjamin has been used as an Anglicized form of Beircheart. However, Benjamin is Hebrew, meaning "son of the right hand." The two names are unrelated, from different origins. It is most likely the two sounded similar, therefore Benjamin was the easiest available Anglicization. There was a Saint Beircheart of Tullylease in Ireland, who was a disciple of Saint Patrick, and in some works Saint Patrick's right hand man is known as Benen. Benen is thought to be Irish for Benjamin. I can't say for sure that Beircheart and Benen from the stories are the same person.
Saint Beircheart's name is often found as Berchert, Berechtir, Beirichtir, Berehert and Beirchert. He was an Englishman, possibly a Saxon prince, who went to Ireland for religious reasons and eventually founded a monastery. The Tullylease Church and Cross, dating to the 6th century, is now a popular tourist destination. The cross, with his name spelled Berachtuine and a note asking people to remember him, is said to be his grave site. (As a side note, there was an Arthurian character named Berchtune, found in "Popular Romances of the Middle Ages" by George W. Cox.)
Bernard is another Anglicization of Beircheart. Bernard is Old German, meaning "bear strong, brave as a bear," and comes from the Anglo-Saxon name Beornheard (a much closer spelling to Beircheart). In Ireland, most often Beircheart = Bernard.
Obviously not much has been published on Beircheart, and there are no records of well known namesakes, but I have found information on two Bernie's who both shared Beircheart as their given Irish name, both called Bernard in all American records. Beircheart has not been used in the U.S.