Sir Cahir was trained by Docwra in martial exercises, in the arts of civility, and in English literature.

He was an apt pupil.

He grew up strong and comely; and he so distinguished himself before he was sixteen years of age in skirmishes with his father's allies, that Sir Henry wrote of him in the following terms: 'The country was overgrown with ancient oak and coppice.

O'Dogherty was with me, alighted when I did, kept me company in the greatest heat of the fight, behaved himself bravely, and with a great deal of love and affection; so much so, that I recommended him at my next meeting with the Lord Deputy Mountjoy, for the honour of knighthood, which was accordingly conferred upon him.' The young knight went to London, was well received at court, and obtained a new grant of a large portion of the O'Dogherty's country.
He married a daughter of Lord Gormanstown, a catholic peer of the Pale, distinguished for loyalty to the English throne, resided with his bride at his Castle of Elagh, or at Burt, or Buncranna, keeping princely state, not in the old Irish fashion, but in the manner of an English nobleman of the period; hunting the red deer in his forest, hawking, or fishing in the teeming waters of Lough Foyle, Lough Swilly, and the Atlantic, which poured their treasures around the promontory of which he was the lord.

His intimate associates were officers and favourites of the king.
Docwra had given up the government of Derry and retired to England.