During his imprisonment in England, James I, King of Scots, fell in love with her and wrote of her in his poem "The Kingis Quair" ('The King's quire-filling poem', or one of 24 sheets---a quire).
Eventually, by the Treaty of London in December 1423, it was agreed that James I should be released for a ransom of œ40,000. In February 1424, before his return to Scotland, he married her and they became the parents of eight children.
After an imprisonment of eighteen years, James I was determined to enforce the rule of law, which meant disaster for those who had ruled in his absence. Resentment grew against his tough style of 
government and, in February 1437 when the King and his court were in residence in Perth, a plot led by Sir Robert Graham caused the murder of one of Scotland's ablest rulers.
Queen Joan, who had been badly wounded while trying to protect her husband, took a terrible revenge. Sir Robert Graham and his fellow conspirators were soon captured and put to death with indescribable
barbarity, unusual even for that cruel age.
In 1439, probably against her will, she married Sir James Stewart, known at "the Black Knight of Lorne", and they produced three more sons.

Source: Leo van de Pas