Gilbert de Clare, 5th Earl of Hertford (1180 – October 25, 1230) was the son of Richard de Clare, 4th Earl of Hertford, from whom he inherited the Clare estates. From his mother, Amice Fitz Robert, he inherited the estates of Gloucester and the honour of St. Hilary, and from Rohese, an ancestor, the moiety of the Giffard estates. In June 1202, he was entrusted with the lands of Harfleur and Montrevillers.

In 1215 Gilbert and his father were two of the barons made Magna Carta sureties and championed Louis "le Dauphin" of France in the First Barons' War, fighting at Lincoln under the baronial banner. He was taken prisoner in 1217 by William Marshal, whose daughter Isabella he later married.

In 1223 he accompanied his brother-in-law, Earl Marshal, in an expedition into Wales.

In 1225 he was present at the confirmation of the Magna Carta by Henry III.

In 1228 he led an army against the Welsh, capturing Morgan Gam, who was released the next year. He then joined in an expedition to Brittany, but died on his way back to Penrose in that duchy. His body was conveyed home by way of Plymouth and Cranbourgh to Tewkesbury. His widow Isabel later married Richard Plantagenet, Earl of Cornwall & King of the Romans. His own arms were: Or, three chevronels gules.

Hertford had six children by his wife Isabella, née Marshal:

Agnes de Clare
Amice de Clare (1220–1287), who married the 6th Earl of Devon
Richard de Clare (1222–1262)
Isabel de Clare (1226–1264), who married the 5th Lord of Annandale
William de Clare (1228–1258)
Gilbert de Clare (b. 1229)

Gilbert de Clare, 5th Earl of Hertford, is our 26th Great Grandfather

Source: Wikipedia