Read the definitions of IR and HRM. IR studies look at the relationships between a workforce and the management of an organization. Academically, students look at employment systems, learning how to analyze and compare different systems. HRM is a larger subject area, which investigates the management's approach organizations adopt to deal with all staff resources, including workers, suppliers and customers.
Look at the relationship between the two. The easiest way to compare the subjects is by understanding how HRM practices and IR are linked. Good HR polices are taken well and liked by people involved in an organization, not prompting IR action. However, decisions that upset or concern people lead to IR. This (in some cases) feeds back into management (alongside government and trade union involvement) and results in them developing more, or changing HR policies, to allow both parties to cooperate effectively with each other.
See how the teaching material differs. There is a natural overlap between both subjects. At undergraduate level they typically fall into a general business program, while at the master's level they are usually taught as a joint program. HRM is considered to be more theoretical, looking at models and theories of how HRM policies are adopted and implemented effectively. IR is more practical, studying the way in which people react and push for alternative treatment, while emphasizing the need to accommodate different parties' interest, values, and needs in the business world.
Study the subject that aligns closest to your chosen career direction. Choosing one of the subjects at college could affect what you end up doing after you finish studying.Those who study HRM have the skills to work in an HR department in a large organization, dealing with issues such as recruitment and staff retention. IR graduates may work for a union or to instead pursue a career in law. Academic interest in both subjects is also growing, allowing you to study either of the two areas professionally in an academic post.