Trim Castle, the largest Norman castle in Ireland, is often known as King John’s Castle. The story behind this “honorific” title is not what you might expect.
When the Normans arrived in Ireland in the late 1100s they claimed the best land in Meath and Dublin, and made Trim the center of their administration north of Dublin. Responsibility for the area was granted to Hugh de Lacy in 1172, and construction began on a fortress on a high point overlooking the River Boyne (which, at that time, was navigable as far as Trim). The castle on the site today, one of the largest in Ireland, was largely built by De Lacy and his son, Walter, and is officially known as Trim Castle. However, many people refer to it as King John’s Castle.
There are at least five castles in Ireland that are known as King John’s Castle, and when you look into the history of this one, there appear to be several other kings who have at least as good a claim on the Castle, if not better: Richard II, Henry II and Henry IV.
Henry II is the one who granted the lordship of Meath to De Lacy — although he later had reason to be concerned over De Lacy’s loyalty. Henry never set foot in the castle.
Richard II actually did stay in Trim Castle in 1399, while he kept out of the way as Henry of Bolingbroke gathered allies upon his return to England. Richard returned to England, where he would soon capitulate peacefully to Bolingbroke (who then became King Henry IV). Richard famously left Henry’s two sons at Trim Castle when he returned to negotiate with their father (they had come under royal protection after Richard had exiled Henry). They were Prince Hal (later Henry V) and his brother, Humphery (later the 1st Duke of Gloucester), about whom Shakespeare later wrote three plays. (They were quartered in the drawbridge tower at the Dublin gate, if you need some trivia to amuse your companions while you explore.)
The real reason that the castle is locally referred to as King John’s may be because in 1210, King John (by the way, yes, this is the King John of Robin Hood fame) became aware that Hugh de Lacy’s heir, Walter de Lacy, was harboring his wife’s mother and brother, who were fugitives from the King’s wrath. Walter had also made some serious improvements to the castle, eliminating several defensive frailties, which may have given John further grounds for suspicion. Acutely aware of the ambitions of the Anglo-Norman lords in Ireland, who would have been only too happy to proclaim themselves king if they could have got away with it, John came to Trim with a great force of men.
Rather than fight the King and enter open rebellion, Walter simply abandoned the castle and left it locked, obliging John to set up camp outside. Ultimately, it all blew over more or less peaceably. Walter was stripped of his lands and titles, only for them to be restored five years later, when John needed allies in his conflict with the English barons.
So, in quintessential Irish fashion, the title King John’s Castle seems to have been applied to Trim Castle as a sarcastic way to make fun of a king who never managed to force his way inside the castle walls.