What I mean by infrastructure here has to do with how Celts moved about in the areas they controlled. In essence, how they crossed land and sea.
It appears the Celts were able to cross land masses with ease via an extensive system of roads. However, the evidence for an extensive system of roads in Celtic controlled areas is largely based on circumstantial evidence. For instance, large wagons with iron rimmed wheels have been found in numerous burial tombs. Thus, one would assume that roads existed to handle these wagons. Little actual evidence of the roads themselves exists because they were made from wood. However, some sections of these wooden roads have been preserved in bogs. It appears the road base was made from birch with less robust tree species being used for the cross planks on the roadway surface.

The case for large ocean going vessels is similar to the case for roads. No vessels have been found that can be attributed to ship building of this nature by the Celts (small river going vessels have been discovered but no large vessels capable of ocean travel have been found). However, the extensive trade between the Celts and other regions indicates that such vessels must have existed. Additionally, various writers also refer to such vessels in their accounts. Julius Caesar, for instance, describes Celtic ships in detail and with a certain amount of envy.

Thus, in this area (surface and water routes) the Celts seem to have had an extensive infrastructure.