  We present optical and near-infrared photometry and spectroscopy of SN
2009ib, a Type II-P supernova in NGC 1559. This object has moderate brightness,
similar to those of the intermediate-luminosity SNe 2008in and 2009N. Its
plateau phase is unusually long, lasting for about 130 days after explosion.
The spectra are similar to those of the subluminous SN 2002gd, with moderate
expansion velocities. We estimate the $^{56}$Ni mass produced as $0.046 \pm
0.015\,{\rm M}_{\sun}$. We determine the distance to SN 2009ib using both the
expanding photosphere method (EPM) and the standard candle method. We also
apply EPM to SN 1986L, a type II-P SN that exploded in the same galaxy.
Combining the results of different methods, we conclude the distance to NGC
1559 as $D=19.8 \pm 3.0$ Mpc. We examine archival, pre-explosion images of the
field taken with the Hubble Space Telescope, and find a faint source at the
position of the SN, which has a yellow colour ($(V-I)_0 = 0.85$ mag). Assuming
it is a single star, we estimate its initial mass as $M_{\rm ZAMS}=20\,{\rm
M}_{\sun}$. We also examine the possibility, that instead of the yellow source
the progenitor of SN 2009ib is a red supergiant star too faint to be detected.
In this case we estimate the upper limit for the initial zero-age main sequence
mass of the progenitor to be $\sim 14-17\,{\rm M}_{\sun}$. In addition, we
infer the physical properties of the progenitor at the explosion via
hydrodynamical modelling of the observables, and estimate the total energy as
$\sim 0.55 \times 10^{51}$~erg, the pre-explosion radius as $\sim 400\,{\rm
R}_{\sun}$, and the ejected envelope mass as $\sim 15\,{\rm M}_{\sun}$, which
implies that the mass of the progenitor before explosion was $\sim
16.5-17\,{\rm M}_{\sun}$.
