Different definitions of disability are used in different contexts – for example to set eligibility for particular services, or to outlaw discrimination on grounds of disability. There is no definitive list of conditions that constitute a disability. Any such list could leave out people with significant but rare conditions. There can also be a wide range of difference between how individuals with a particular condition are affected, ranging from mild to severe difficulties. A person’s environment, which includes the supports they have and the physical or social barriers they face, influences the scale of the challenges they face in everyday life.

The Disability Act 2005 set out the following definition:
“disability”, in relation to a person, means a substantial restriction in the capacity of the person to carry on a profession, business or occupation in the State or to participate in social or cultural life in the State by reason of an enduring physical, sensory, mental health or intellectual impairment

The Equality Acts (Employment Equality Acts and the Equal Status Acts), which outlaw discrimination on grounds of disability, use a wider definition, and cover past as well as current disability:
"Disability means:
(a) the total or partial absence of a person’s bodily or mental functions, including the absence of a part of a person’s body;
(b) the presence in the body of organisms causing, or likely to cause, chronic disease or illness;
(c) the malfunction, malformation or disfigurement of a part of a person’s body;
(d) a condition or malfunction which results in a person learning differently from a person without the condition or malfunction; or
(e) a condition, disease or illness which affects a person’s thought processes, perception of reality, emotions or judgement or which results in disturbed behaviour."

Census 2011, and other official surveys, used the following definition of disability:
A person with one or more of the following long-lasting conditions or difficulties:
- Blindness or a severe vision impairment
- Deafness or a severe hearing impairment
- An intellectual disability
- A difficulty with learning, remembering or concentrating
- A difficulty with basic physical activities
- A psychological or emotional condition
- A difficulty with pain, breathing, or any other chronic illness or condition