Information Architecture is a more specialized role than User Experience Designer. They can very well work together in the same project where the UXD deals with everything but IA core competences. You can look at the UXD as the project lead in IA related questions where IA deals with core IA tasks.
Differences between Information Architecture and User Experience Design
Jesse James Garret, author of "The Elements of User Experience", made an infamous visualization on User Experience elements before writing his book. It has two different scales: Abstract to Concrete- scale and Conception to Completion-scale. It's divided into five layers which should be read from the bottom and upward (from Conception to Completion and from Abstract to Concrete over time).
Download the original PDF from March 30, 2000.
It reflects the process of designing any artifact from a User Experience perspective. Garret highlights what is important at each layer and each stage of the project. Beware though, this image should not be seen as a project roadmap where you complete one layer before moving on to the next. Garret states though that you can't finish one layer before preceding layers are finished. You need to be agile and move between layers as you move along.
It is possible to transfer these layers into different competencies. The User Experience Designer cover all of these aspects of the visualization:
- Site Objectives and User Needs
- Functional Specification and Content Requirements
- Interaction Design and Information Architecture
- Information Design, Interface Design and Navigation Design
- Visual Design
The Information Architect (IA) is a more specialized area of User Experience. In short an Information Architect "connects people to the content they're looking for". To do this the IA have four different tools:
- Classification and Hierarchy
- Labels and Tagging
- Navigation and Wayfinding
Image by murdocke23 on Flickr
That's the difference, and similarities, between a User Experience Designer and an Information Architect.