  We present an experimental investigation of the magnetic ground state in
Ba$_4$NbIr$_3$O$_{12}$, a fractional valent trimer iridate. X-ray absorption
and photoemission spectroscopy show that the Ir valence lies between 3+ and 4+
while Nb is pentavalent. Combined dc/ac magnetization, specific heat, and muon
spin rotation/relaxation ($\mu$SR) measurements reveal no magnetic phase
transition down to 0.05~K. Despite a significant Weiss temperature
($\Theta_{\mathrm{W}} \sim -15$ to $-25$~K) indicating antiferromagnetic
correlations, a quantum spin-liquid (QSL) phase emerges and persists down to
0.1~K. This state likely arises from geometric frustration in the edge-sharing
equilateral triangle Ir network. Our $\mu$SR analysis reveals a two-component
depolarization, arising from the coexistence of rapidly (90\%) and slowly
(10\%) fluctuating Ir moments. Powder x-ray diffraction and Ir-L$_3$edge x-ray
absorption fine structure spectroscopy identify ~8-10\% Nb/Ir site-exchange,
reducing frustration within part of the Ir network, and likely leading to the
faster muon spin relaxation, while the structurally ordered Ir ions remain
highly geometrically frustrated, giving rise to the rapidly spin-fluctuating
QSL ground state. At low temperatures, the magnetic specific heat varies as
$\gamma T + \alpha T^2$, indicating gapless spinon excitations, and possible
Dirac QSL features with linear spinon dispersion, respectively.
