  This thesis is devoted to the study of the interactions existing between the
algebraic structure of locally compact groups and the properties of their
continuous unitary representations, with a special emphasis on the Type I
groups. On the one hand, the thesis provides a general overview of the theory
of unitary representations of locally compact groups aswell as an overview of
the classification of the irreducible unitary representations of the full group
of automorphisms of any semi-regular tree. On the other hand, it contains
various of our personal contributions to this domain of mathematics. Among
other things, we provide an algebraic characterisation of certain Lie groups
that are irreducibly represented such as the connected nilpotent Lie groups.
Concerning totally disconnected locally compact groups, we expose an axiomatic
framework to describe their irreducible representations all of whose isotropy
groups are "small". We provide concrete applications of this machinery to
groups of automorphisms of trees and right-angled buildings leading to a
substantial contribution to Nebbia's CCR conjecture. We also provide a
description of the unitary dual of the groups of automorphisms of trees acting
2-transitively on the boundary and whose local action at every vertex contains
the alternating group and describe the Fell topology of this space.
