Upon the death of his father, his younger but legitimate half-brother, Sir Dermot Ó Seachnasaigh, inherited Cenél Áeda.
Contention between the two brothers continued till 1585, when they both travelled to Dublin to attend the 1585 Parliament.
It seems that the parliament found in favour of Sir Dermot, which left John destitute.
The Annals of the Four Masters, sub anno 1601, relates John's fate: An unusual accident and a sad fatality occurred to the camp of the Bourkes, namely, an advantage was taken of their want of watching, so that their enemies came into the midst of them.
They left them lying mangled and slaughtered, pierced and blood-stained corpses, throughout their tents and booths.