Doonmore Castle, the big fort, adjacent to the harbour, was built by Philip Mac Sheeda Nor McCon.  The annuals of the Four Masters refer to it as Dun More Mhic and Fhearnachaigh.  At the end of the 16th Century, Donough Mac Dermot Mac an Fhearmachaigh was in possession but Mahon MacGorman held title to one third of the castle and all of Donough's Land. Sir Daniel O'Brien of Dough was listed as the owner in 1574.  In 1594 Mahon MacGorman assigned his interest in the place to the Earl of Thomond.

Murrough MacGucarrick  claimed the castle in 1619.  Doonmore was confiscated by the Crown in 1688 and sold in 1703. It was inhabited during 1808 but classed as a ruin in 1837.

The original castle was considerably higher until a turret at the south-western corner fell soon after 1898 bringing most of the walls of the upper room with it.   The rest was pulled down soon afterwards leaving only the lower portion, from the stone vault down, intact.