  We study isomorphism classes of untwisted irregular singular meromorphic
connections on principal bundles over (wild) Riemann surfaces, for any complex
reductive structure group $G$ and polar divisor. In particular we compute the
stabilisers of suitable marked points on their principal part orbits, showing
the stabilisers are connected and controlled by the corresponding filtration of
(Levi factors of) nested parabolic subgroups of $G$; this uniquely determines
the orbits as complex homogeneous manifolds for groups of jets of principal
$G$-bundle automorphisms. Moreover, when the residue is semisimple we stratify
the space of orbits by the stabilisers, relating this to local wild mapping
class groups and generalising the Levi stratification of a Cartan subalgebra
$\mathfrak{t} \subseteq \mathfrak{g} = \operatorname{Lie}(G)$: the dense
stratum corresponds to the generic setting of irregular isomonodromic
deformations \`a la Jimbo-Miwa-Ueno.
  Then we adapt a result of Alekseev-Lachowska to deformation-quantise
nongeneric orbits: the $\ast$-product involves affine-Lie-algebra modules,
extending the generalised Verma modules (in the case of regular singularities)
and the `singularity' modules of F.-R. (in the case of generic irregular
singularities). As in the generic case, the modules contain Whittaker vectors
for the Gaiotto-Teschner Virasoro pairs from irregular Liouville conformal
field theory; but they now provide all the quotients which are obtained when
the corresponding parameters leave the aforementioned dense strata. We also
construct Shapovalov forms for the corresponding representations of truncated
(holomorphic) current Lie algebras, leading to a conjectural irreducibility
criterion. Finally, we use these representations to construct new flat vector
bundles of vacua/covacua \`a la Wess--Zumino-Novikov-Witten, equipped with
connections \`a la Knizhnik-Zamolodchikov.
