  We investigate the different facets of ion acceleration by a relativistically
intense circularly polarized laser pulse interacting with thin near-critical
density plasma targets. Our simulations establish that plasma density gradient
and laser frequency chirp can be controlled to switch the interaction from the
transparent to the opaque regimes of operation. This enables one to choose
between a Maxwellian like ion energy distribution with a cut-off energy, in the
relativistically transparent regime, or a quasi-monoenergetic spectrum, in the
opaque regime. We subsequently demonstrate that a double-layer multi-species
target configuration, can be effectively utilized for optimal generation of
quasi mono-energetic ion bunches of a desired species. We finally demonstrate,
the feasibility of generating mono-energetic proton beams with energy peak at
$\mathcal{E}\approx20\sim40$ MeV with a narrow energy spread of $\Delta
\mathcal{E}/\mathcal{E}\approx18-28.6\%$ confined within a divergence angle of
$\sim 175$ millirad at a reasonable laser peak intensity of $I_{0}\simeq
5.4\times 10^{20}\, \mathrm{W/cm^2}$.
