Regular readers of this blog will know about Margaret de Clare, Edward II's niece, who married Piers Gaveston and Hugh Audley. What is not nearly as well-known is that there were two more Margaret de Clares of the era, the aunt and cousin of the famous Margaret de Clare. Here's a post about them.
Margaret de Clare, the aunt
Margaret was born in 1249 or 1250, daughter of Richard de Clare, earl of Gloucester (1222-1262) and Maud de Lacy, daughter of the earl of Lincoln (died 1288/89). Her siblings were: Gilbert 'the Red', earl of Gloucester; Thomas, lord of Thomond; Bogo, a rich and scandalous cleric; Isabel, who married William VII, marchese of Montferrat; Rohese, who married Sir Roger Mowbray; and the oddly-named Eglentina, died young.
On 6 October 1272, Margaret married Edmund, earl of Cornwall, at Ruislip in Middlesex. Edmund was born on 26 December 1249, son of Henry III's brother Richard of Cornwall and Sanchia, sister of Eleanor of Provence - which means that he had all four grandparents in common with Edward I, and was also the first cousin of Philip III of France and of Charles 'the Lame', king of Sicily. Edmund succeeded his father as earl of Cornwall on Richard's death in April 1272, his elder half-brother Henry of Almain having been murdered in Italy in March 1271 by two of Simon de Montfort's sons.
Unfortunately, Margaret and Edmund's marriage proved utterly disastrous. Although Margaret was pregnant in January 1285, she must either have miscarried or suffered a stillbirth, and the couple's childlessness may have contributed to their awful marital difficulties.  Margaret accused of Edmund of cruelty and neglect and even alleged that she went in fear of her life from him - whether that's true or not, I don't know.  Certainly Edmund refused to cohabit with Margaret - at least from 1285 onwards - and their marriage became the subject of a papal investigation in 1289. The following year, John Pecham, archbishop of Canterbury, excommunicated Edmund. The couple officially separated in 1294 and Edmund granted Margaret lands worth £800 annually for the rest of her life, while Margaret took vows of chastity, to last until Edmund's death.
Edmund died shortly before 25 September 1300 at the age of fifty, and his heart and flesh were buried in early 1301 at Ashridge Priory in Hertfordshire, which he had founded in 1283. Sixteen-year-old Edward of Caernarfon attended the funeral, representing his father Edward I. Edmund's bones were later buried at Hailes Abbey in Gloucestershire, which his father Richard had founded in 1246 and where he (Richard) and Edmund's mother Sanchia were buried. Countess Margaret continued to live quietly and somewhat obscurely, and never remarried. In 1303, she lent £69 to her nephew Gilbert de Clare - son of Margaret's brother Thomas, lord of Thomond - and in March 1308, was granted all the 'liberties' that her former husband and his father "were wont to use in their lands" by Edward II. 
On 1 November 1307, Margaret's niece married Piers Gaveston and became countess of Cornwall, so that there were, confusingly, two Margaret de Clares, countesses of Cornwall. On 5 August 1309, after Piers Gaveston had returned from his second exile and was restored to the earldom, Margaret was ordered to "render fealty" to Piers and her niece for the dower lands she held in Rutland.  Margaret was living at Harewell in Berkshire in late 1311, when Queen Isabella sent her letters there. Isabella also dispatched "various precious goods" to Margaret's very pregnant niece Margaret Gaveston (Isabella's niece by marriage) at Wallingford.  Margaret died shortly before 16 September 1312, in her early sixties, and was buried at Chertsey Abbey. The lands granted to her by Edmund passed to Edward II; not because he was king, but to him personally, as Edmund of Cornwall's heir - because Edmund had no children, nieces or nephews, his nearest male relative was his first cousin Edward I, and then Edward II. The order to the escheator this side Trent calls them "the lands which Margaret, countess of Cornwall, deceased, held in dower of the king's inheritance." Edward granted Margaret's lands immediately to Margaret Gaveston, recently widowed, to hold for her sustenance until he could make other provision for her. 
Margaret de Clare, the cousin
This Margaret was the daughter of Thomas de Clare, lord of Thomond and Juliane Fitzgerald or Fitzmaurice, and was the niece of Gilbert 'the Red' and Margaret de Clare, above, and the first cousin of Margaret de Clare Gaveston. She was born on or about 1 April 1287 at Bunratty Castle in Ireland. Her brother Gilbert (born 1281) married Hugh Despenser the Elder's daughter Isabel in about 1306, was a member of Edward II's household before he became king, and died childless in November 1307. Her other brother Richard died in June 1318, leaving a baby son Thomas. Thomas died in April 1321, which left Margaret and her sister Maud - wife of Robert Clifford, who was killed at Bannockburn in 1314 - as heirs to the lordship of Thomond.
Margaret married firstly Gilbert de Umfraville, son of the earl of Angus, with whom she had no children. Gilbert died in about 1303. She married secondly, sometime before June 1308, Bartholomew, Lord Badlesmere, an influential knight in the retinue of Margaret's first cousin Gilbert de Clare, earl of Gloucester, who became steward of Edward II's household in 1318. The couple had five children: Giles, Elizabeth, Margery, Maud and Margaret.
In 1319, Margaret was staying at Cheshunt in Hertfordshire, when she had the misfortune to be held hostage by a group of more than sixty men and women, who demanded a ransom of £100. She was imprisoned overnight until a knight in shining armour rode to her rescue the following day – Hugh Despenser the Younger.  Edward II imprisoned twenty of the men responsible at Hertford Castle and then at the Tower of London when Bartholomew Badlesmere was still his loyal steward, but in November 1321, with Bartholomew now his enemy, he pardoned them. 
After the Despenser War in 1321, when the Marcher lords attacked the Despensers' lands in Wales and England, Margaret's husband Bartholomew - presumably angry at the Despensers' dominance over Edward II, which drastically limited his influence - switched sides and joined the Marchers. This would prove to be a tragic decision; Edward thereafter loathed Bartholomew, and the earl of Lancaster, leader of the opposition to Edward and the Despensers, loathed him already. Margaret Badlesmere is most famous for being the person who refused Queen Isabella entry to Leeds Castle in October 1321, which gave Edward the excuse he needed to mount a campaign against the Marchers.
Edward II, never a man to forgive and forget a betrayal, pointedly excluded Bartholomew by name from the safe-conducts he offered the other Marchers in early 1322.  Following the Marchers' defeat at Boroughbridge on 16 March 1322, Bartholomew sought refuge at his nephew the bishop of Lincoln's manor of Stowe Park, but was captured by Edward's Scottish friend Donald of Mar and died a terrible death in April 1322 - dragged the three miles from Canterbury to the crossroads at Blean, hanged, drawn and quartered.  The events of 1322 hit Margaret hard: not only did her husband suffer grotesque execution, but her nephew Roger Clifford was hanged in York in March 1322, as was her first cousin John Mowbray (son of Rohese de Clare, sister of Margaret's father). However, Margaret's brother-in-law Sir Robert de Welle - who had married her widowed sister Maud Clifford in 1315 - remained loyal to the king, until at least August 1326, and Edward granted 'the wardenship of the hospital of St Nicholas, Pontefract' to Robert Woodhouse at Welle's request the day after his stepson Roger Clifford's execution. 
Margaret and her young children, and her husband's adult nephew Bartholomew Burghersh, were imprisoned at Dover Castle then the Tower of London following the surrender of Leeds Castle on 31 October 1321. She was released on 3 November 1322, and went to live at the convent of the Minorites at Aldgate; Edward II granted her two shillings a day for her sustenance. On 1 July 1324, Edward granted Margaret permission "to go to her friends within the realm whither she will, provided that she be always ready to come to the king when summoned," and continued to give her two shillings a day. 
Margaret died sometime between 22 October 1333 and 3 January 1334, at the age of forty-six. Her only son Giles, born on 18 October 1314, married Elizabeth Montacute, daughter of Edward III's great friend the earl of Salisbury, and died childless on 7 June 1338, leaving his four sisters as his co-heirs. Margaret's daughter Elizabeth married firstly Roger Mortimer's eldest son and heir Edmund, then Edward II's nephew William de Bohun; Margaret de Clare Badlesmere was thus the grandmother of Roger Mortimer the Younger, second earl of March (1328-1360), and of Humphrey de Bohun, earl of Hereford and Northampton (1342-1373). Through her daughter Maud, Margaret was also the grandmother of Thomas de Vere, eighth earl of Oxford (c. 1336-1371) and Aubrey de Vere, tenth earl of Oxford (c. 1338-1400), and the great-grandmother of Richard II's notorious favourite Robert de Vere, ninth earl of Oxford and duke of Ireland (1362-1392).
1) Oxford Dictionary of National Biography, Edmund of Cornwall entry.
2) Jennifer C. Ward, English Noblewomen in the Later Middle Ages (1992), p. 31; Michael Altschul, A Baronial Family in Medieval England: The de Clares, 1217-1314 (1965), pp. 35-36.
3) Calendar of Close Rolls 1301-1307, p. 89; Cal Close Rolls 1307-1313, p. 55.
4) Calendar of Patent Rolls 1307-1313, p. 187.
5) The Household Book of Queen Isabella of England, for the fifth regnal year of Edward II (8 July 1311 to 7 July 1312), ed. F. D. Blackley and Gustav Hermansen (1971), p. 139.
6) Calendar of Fine Rolls 1307-1319, p. 146; Cal Pat Rolls 1307-1313, pp. 497, 502.
7) Cal Pat Rolls 1317-1321, p. 473.
8) Cal Close Rolls 1318-1323, p. 267; Cal Pat Rolls 1321-1324, p. 37.
9) Cal Pat Rolls 1321-1324, pp. 47-48, 51, 70, 71, 76.
10) The Brut, ed. F. W. D. Brie, vol. 1, p. 221. Le Livere de Reis de Britanie e le Livere de Reis de Engletere, ed. John Glover, pp. 341-343, says Bartholomew was captured "in a small wood near Brickden."
11) Cal Pat Rolls 1321-1324, p. 85, and see also p. 193 and Cal Pat Rolls 1324-1327, p. 315.
12) Cal Close Rolls 1318-1323, pp. 604, 627; Cal Close Rolls 1323-1327, pp. 46, 48, 120, 236.