This Margaret Beaufort was one of five daughters of Edmund Beaufort, Duke of Somerset, and Eleanor Beauchamp. Her father was killed at the first battle of St. Albans on May 22, 1455. All three of her brothers met violent ends during the Wars of the Roses: the eldest, Henry Beaufort, was executed by Yorkist troops in 1464; the second, Edmund Beaufort, was executed after the battle of Tewkesbury; and the third, John, died in battle at Tewkesbury.
Margaret married Humphrey Stafford, Earl of Stafford, the eldest son of Humphrey Stafford, the first Duke of Buckingham. According to a manuscript cited by Carole Rawcliffe, the marriage took place in 1444. Probably Margaret was a child when she married, as her eldest son was not born until 1455.
Margaret was pregnant with her first son, Henry Stafford, when her father was killed at St. Albans. Many sources, even the Complete Peerage, mistakenly claim that the Earl of Stafford, Margaret's husband, was killed at St. Albans as well, but he, like his father the Duke of Buckingham, survived the battle with wounds. Margaret and Humphrey's eldest son, Henry, was born on September 4, 1455.
In 1458, the the Earl of Stafford died of the plague. He had been appointed in 1457 to the council of Prince Edward, son of Henry VI and Margaret of Anjou. Margaret Beaufort was left a widow with two sons, Henry and Humphrey.
The first Duke of Buckingham was killed at Northampton in 1460, making Margaret's son Henry the second duke. His wardship and marriage were in the hands of the first duke's widow, Katherine, and the Archbishop of Canterbury, Thomas Bourchier. Later, Edward IV purchased the wardship, and Henry and his younger brother were eventually brought into the household of Elizabeth Woodville. Humphrey, along with Henry, was made a Knight of the Bath and shared Henry's tutor, but he apparently died young, for there is no trace of him after the 1460's.
Margaret Beaufort, meanwhile, remarried. Her second husband was Richard Darell, variously spelled as Darelle, Darrell, and Dayrell, who was a younger son in a family of five brothers. The two had a daughter, Margaret.
It is here where things get murky with Margaret. The Collections for a History of Staffordshire contains this entry:
Wilts. Richard Darelle, late of Litelcote, armiger, was summoned at the suit of Alexander Darelle, executor of the will of Elizabeth Darelle, in a plea that he should pay to him a sum of 45 marks, which he unjustly withheld, and Alexander stated that on the 1st August, 3 E. IV,  the said Richard had placed Margaret, the Countess of Stafford, to board ("ad mensam") with the said Elizabeth during her lifetime, paying to her for each week for which the Countess was at board with her the sum of 13s. 4d. for her diets ("pro dietis suis"), such rate to continue during the whole time of the presence of the said Countess ("essencie ipsius Comitesse") at board with Elizabeth, and notwithstanding the said Countess had remained at board with her from the said 1st August for the 45 weeks following, the said Richard had not paid Elizabeth during her life for the said arrears, and had refused to pay her executor, notwithstanding frequent requisitions made upon him, and he produced the testamentary letters of the said Elizabeth, which satisfied the Court that he was her executor. Richard Darelle appeared by attorney, and denied that he had detained the money, as stated by Alexander, and appealed to a jury which was to be summoned for the Quindene of Easter Day. A postscript shews that the Sheriff had made no return to the writ up to Michaelmas term, 6 E. IV. m. 341, dorso.
The editor concludes that this entry indicates that Margaret was an "imbecile." So was she actually incapacitated, mentally or physically? It's difficult to say for sure from this single entry, but it does sound as if Margaret was being made the responsibility of Elizabeth (apparently her mother-in-law, Elizabeth Calston, who died in 1464) rather than as staying with her as an ordinary guest. It has been suggested that Margaret was staying with Elizabeth during her pregnancy, but "such rate to continue during the whole time of the presence of the said Countess" suggests a more indefinite term.
Whatever the reason the countess was staying with Elizabeth, almost nothing else is heard of her. She is noted as presenting a rector, John Southwell, to Wells in Norfolk in 1463; in 1465, Richard Darell presented William Dudley to Wells in Margaret's right. (Dudley was later Bishop of Durham.) British History Online (Romsey) indicates that she had manors at Stanbridge Earls, Wexcombe, and Bedwyn. (See text by note 206.) Even her death date is unclear. Some sources give the date as being in the 1470's, and others give it in 1480, but the latter appear to be confusing her with her Stafford mother-in-law, Katherine Neville, who died that year. She was certainly dead by May 22, 1481, when Richard Darell and his second wife, Jane Baron, deeded over some land. Thus, she was spared the knowledge of the beheading of her only surviving son on November 2, 1483.
Some of the countess's obscurity might be explained by her Beaufort connections: as her father and all of her brothers died in the service of the House of Lancaster, she was unlikely to be welcome at Edward IV's court or to relish going there. Nonetheless, the countess's second husband, Richard Darell, was not out of favor with Edward IV, but was appointed to a number of commissions. He also served as one of his stepson's councilors and in 1483 was present at the coronation of Richard III. Having steered clear of his stepson's rebellion later in 1483, Richard Darell died in 1489. In 1480, he and John Tuchet, Lord Audley, arranged for Audley's heir, James Tuchet, to marry Darell's daughter by Margaret Beaufort, named Margaret after her mother. It was a short-lived marriage, for by 1488, James Tuchet had remarried. Like her mother, Margaret Darell was spared grief by her early death, for James Tuchet was beheaded in 1497 after rising against Henry VII.