With the attainder of the Earl of Kildare and
following rebellion, Conn sided with his in-laws the FitzGeralds. An alliance referred to as the
Geraldine League sought the restoration of the heir of the FitzGerald lordship without interference from King Henry VIII of England. That rebellion was stoked by the idea of casting off England's Protestant church in Ireland. In 1539 Conn Bacagh and
Manus O'Donnell attacked
The Pale. They were returning to their territories with treasure and spoils when
Lord Deputy Leonard Grey overtook them at the
Ford of Belahoe, south of
Carrickmacross. The O'Neills and O'Donnells were quickly overwhelmed and suffered 400 casualties before fleeing in disarray and leaving their treasure and spoil. the newly established
King of Ireland. This was part of a major policy of
surrender and regrant in which Gaelic lords formally acknowledged the Crown's authority. After Tyrone was invaded in 1541 by Sir
Anthony St Leger, the lord deputy, Conn and the Geraldine League were defeated and he made his submission. Conn delivered up his son
Phelim Caoch O'Neill as a hostage. In early 1542, Phelim was killed with one cast of a javelin by MacDonnell gallowglass according to the entry recording his death in the Annals of the Four Masters of Ireland., just prior to his father's submission to Henry VIII. Conn attended a parliament held at
Trim, and, crossing to England, became a Protestant, and made his submission at Greenwich to
Henry VIII. ==Civil war==