Background: Homeless people have poor health and mortality indices. Despite this they make poor usage of health services. This study sought to understand why they use health services differently from the domiciled population.

Methods: Ethnographic observations were conducted at several homeless services, in Dublin. This was supplemented with 47 semi-structured interviews with homeless people and two focus groups of homeless people and hospital doctors. A critical-realist approach was adopted for interpretation of the data.

Results: Homeless people tended to present late in their illness; default early from treatment; have low usage of primary-care, preventative and outpatient services; have high usage of Emergency and Inpatient services; and poor compliance with medication. They tended to avoid psychiatric services. A number of external barriers were identified. These were classified as physical (distance) administrative (application process for medical care; appointments; queues; the management of addiction in hospital; rules of service; and information providing processes); and attitudinal (stigma; differing attitudes as to appropriate use of services. A new form of barrier, Conversations of Exclusion was identified and described. Internalised barriers were identified which were in nature, either cognitive (fatalistic, denial, deferral to future, presumption of poor treatment or discrimination, self blame and survival cognitions) or emotional (fear; embarrassment, hopelessness and poor self-esteem). Generative mechanisms for these factors were identified which either affected participants prior to homelessness (marginalization causing hopelessness, familial dysfunction, substance misuse, fear of authority, illiteracy; mental health; and poor English) or after becoming homeless (homelessness; ubiquity of premature death; substance misuse; prioritization of survival over health; threat of violence; chaotic nature of homelessness; negative experiences of authority; and stigma.

Conclusions: An explanatory critical realist model integrating the identified generative mechanisms, external and internalised barriers was developed to explain why the Health service Utilization of homeless people differs from the domiciled populations. This new model has implications for health service policy makers and providers in how they design and deliver accessible health services to homeless people.