She was the fourth and final wife of Hugh O'Neill, Earl of Tyrone, a leading Gaelic lord in Ireland during the late Elizabethan and early Stuart eras.

Catherine was part of the Magennis dynasty, a powerful family in County Down which enjoyed favour from the Crown.

She was separated from her children and settled with her husband in Rome, receiving minimal support from Philip III of Spain and Pope Paul V. After her husband's death, Catherine faced major financial difficulties and constantly petitioned the Spanish government for their assistance.

The unification between the Magennis family and confederacy leader Hugh O'Neill was sealed with Catherine's betrothal to O’Neill.

[21] Reportedly O’Neill considered "putting away" Catherine in December 1605, and it was rumored that he had assembled a group of clerics to explore his options.

[1] In response to this, Catherine allegedly confronted her husband, warning him that if he didn't stop his abuse, she "would discover him so far as to infer again to rebellion or to lose his head."

[27] English administrator Arthur Chichester heard about the strain on their marriage, and sent officer Toby Caulfield to recruit Catherine as a double agent.

[21] In September 1607, facing potential arrest for treasonous activities,[26] Catherine's husband made a "snap decision" to leave Ireland for continental Europe, along with ninety of his followers.

[32] In Rome, the couple received a small pension from Pope Paul V.[28][34] Lord Deputy Arthur Chichester placed Conn in the custody of English officer Sir Tobias Caulfeild.

Catherine asked to return to Leuven to reunite with her children, but her petitions were constantly denied by Spanish authorities.

[32] When Catherine pleaded for her health, frequently complaining about the "city's air", the authorities suggested she go to Naples or Sicily.

Robert Lombard was a spy for the Crown and may have attempted to get information from the Countess about her husband, which he relayed on to London and Dublin.

"[42][37] The unhappy retainers asked the late earl's secretary, Henry O'Hagan, to inform Shane that his mother was refusing to give them the money bequeathed to them.

They cautioned Shane to send someone to Rome, to deposit his late father's money and valuable in a bank before Catherine could.

Historian Jerrold Casway believes this saga exposes the Irish refugees' "extreme dependency", as well as Catherine's estrangement from her husband and his retainers.

The late Earl's trusted friend and former Spanish ambassador to Rome, Count de Castro, asked Philip III of Spain for compassion towards Catherine's "miserable state, alone and without property in a foreign land... with no other protection than that of Your Majesty.

[36] In September 1616,[43] he directed Castro's successor, Cardinal Borja y Velasco, to grant Catherine the same allowance as her late husband, to the frustration of O'Neill's dependents.

She eventually escaped to Naples to evade creditors, to alleviate her poor health and to be closer to her new benefactor Pedro Téllez-Girón, 3rd Duke of Osuna.

On 22 June 1618, Catherine described herself to Philip III as "this afflicted and unprotected widow," who feared "less to die of hunger than to become the ridicule of the English."

Her fate highlights the extreme dependency faced by the Irish refugees and embodies the high-risk politics typical of her husband's marriages.