Her influence over the reign of her second husband Turlough Lynagh O'Neill, Lord of Tír Eoghain, is noted by both contemporary and modern sources.
[7][3][8] In June 1563, Campbell gave Mary, Queen of Scots a "marvellous fair" Highland costume to wear on a progress to Argyll.
[2][10] Now a widow, Campbell commanded the loyalty of a substantial number of Scots mercenaries, who were drawn to Ireland due to changes in Scotland.
[1][12] According to historian Judy Barry, it seems Turlough "settled into a subordinate role, accepting both Agnes's judgement and her superior diplomatic skills".
At her request, Turlough agreed to an accord with the English government in 1571, and in June 1575, she negotiated peace terms with the 1st Earl of Essex.
[1] Campbell feared that Turlough's rival Sorley Boy MacDonnell, also James MacDonald's brother, was a threat to her sons' political success.
She aimed to establish her sons Angus and Donald Gorm MacDonald as rivals to Sorley Boy by claiming land for them in the Glens of Antrim.
[1] Campbell was finally successful when in May 1586, her and Angus were granted "Bissett's lands" - estates in the Glens of Antrim long claimed by Sorley Boy.
The same year, she also entered discussions with Lord Deputy John Perrot about restoring lands leased by her husband to Hugh O'Neill, Earl of Tyrone.