From "Complete Peerage England, Scotland, Ireland, Great Britain" by George E Cokayne, CS421.G2 1982, v IV, page 236, Desmond:
Thomas fitz Morice Fitzgerald married before 7 Feb 1283, Margaret, daughter of Sir Thomas de Berkeley of Berkeley, county Gloucester [Lord Berkeley], by Joan, daughter of William de Ferrers, Earl of Derby. The fact that Margaret was the King's cousin, restricts the inquiry as to her paternity to a very few families. Thomas fitz Morice made a fine with the King in 700 marks for his marriage, and Thomas de Berkeley bound himself to pay the whole of this sum. Fine Rolls, 12 Edw. I, m. 18: Close Roll, 12 Edw I, m. 4 d. Thomas de Berkeley had a daughter Margaret, to whom no husband has hitherto been assigned. The charter of 6 Feb 1291/2 is still among the muniments at Berkeley Castle. Further evidence is given elsewhere in these notes of a close connection between the Berkeleys and Thomas fitz Morice, his wife, and his sons. It may therefore be confidently asserted that Margaret, wife of Thomas fitz Morice, was daughter of Thomas de Berkeley, and consequently granddaughter of Isabel, called the King's cousin, daughter of Richard de Dovor, illegitimate son of King John. The statement in the pedigress that the wife of Thomas fitz Morice was "Margaret de Burgh" [also called Burke] can be readily explained. They mistook the abbreviation of Berk' for Burk'. In Aug 1311, at the request of Thomas de Berkele, Reynold and Margaret [Thomas's daughter and son-in-law] were given leave to pay a debt to the King of £300 by yearly instalments of £30. Close Roll, 5 Edw. II, m. 31.
On 9 Jul 1298 Thomas de Berkele mainperned to render up when required the son and heir of Thomas fitzMorice, a minor [and de Berkeley's grandson], if the King should deliver him to Margaret his mother, the King's cousin, and late the wife of the said Thomas fitz Morice. Close Roll, 26 Edw. I, m. 6. The marriage of Thomas, son and heir of Thomas fitz Morice of Ireland was granted to Thomas de Berkeleye the elder, 7 Feb 1300/1. Patent Roll, 29 Edw. I, m. 29.

Weis" "Ancestral Roots. . ." (26:29), (59:30). Cokayne's "Complete Peerage" (Berkeley, pp. 127-128). Called "Thomas the wise". Though underage, he was present at the Battle of Eversham, where the future KING EDWARD I defeated the forces of Simon de Montfort (RIN 2884*). For nearly every year of his last 50, he was "employed either against the Welsh, Scots, or the French." He was made Vice-Constable of England in 1279, was at the bloody battle and defeat of the Scots at Falkirk 1298, the siege of Carlaverock 1300, and was taken prisoner at the Battle of Bannockburn 1314, paying a large sum for his ransom. He was on the commission to examine the claims to the Crown of Scotland 1292, and was on an embassy to France 1296 and to Pope Clement V 1307.

This lord was one of the most remarkable men of his age. Smyth calls him-
"A man of men; a man for all hours and all affairs; a man at home and abroad, in peace and in war, in the foreign embassies of his Prince, and, in his country governments, of an universal understanding. And for his private husbandries and house keepings he admitted of few compeers. A wise, devout, and honest lord, much to be preferred above the best of his six forefathers."

After his succession to the Barony he devoted himself very much to the management and improvement of his estates, keeping many of his manors in his own hands, of which most minute and accurate accounts were kept, showing how the demesne lands were stocked and farmed, and how the produce was disposed of. Like several of his predecessors he granted away much land in fee, reserving what was then the full annual value as a chief rent; the object of this was to maintain the revenue of the estate at its then value, thinking that from the disturbed state of the kingdom it was more likely to diminish than to increase. His standing household consisted of upwards of 300 persons, of the various ranks of knights, esquires, yeomen, grooms, and pages, besides of others of less degree.

Lord Berkeley's public, civil, and military employments were as numerous as his domestic engagements. From the battle of Evesham in 1265, to 1319, he was almost constantly in arms and served in nearly every engagement in the civil wars, as well as against the French, Scots, and Welsh, during that turbulent period. In 1295 he was sent as ambassador to the king of France. In 1307, he was appointed with the Bishop of Worcester to go on an embassy to Rome, but their mission was stopped by the death of the king (Edward I) at Carlisle. Lord Berkeley was present at the coronation of Edward II and soon afterwards went with his two sons Maurice and John to France to witness the king's marriage with the Princess Isabella, little thinking probably, to what a tragedy that marriage would lead, and how great a share his family were destined to take in it! At the disastrous battle of Bannockburn, lord Berkeley and his son Thomas were both among the prisoners, but Maurice escaped, and aided in effecting the ransom of his father and brother. In 1319, lord Berkeley was again in arms, though 74 years of age, and joined the royal army at Newcastle with his son Maurice and Maurice’s two sons, there being thus three generations of Berkeleys in the field at once; this was Thomas lord Berkeley's 28th campaign and it was his last. After his return home he was several times written to by the king, Edward II, requiring him to repress the local and partial insurrections which were caused by the discontents occasioned by the King's weakness and incapacity and his devotion to favourites.

Thomas, 6th lord Berkeley, died in 1321, and was buried with his forefathers in St. Augustine’s under an arch between the vestry and the south aisle.