A national survey of long-term residential care has found that just 11% of nursing homes have dedicated dementia care units.
Half of all specialist care units are in five counties.
The Trinity College Dublin/St James's Hospital study found that Sligo, Wicklow, Carlow, Kilkenny, Westmeath and Offaly appear to have no units.
It also found excessive numbers of patients in individual units.
A survey of nearly 470 nursing homes for the study found that the overall average number of residents in units was 19, but could be as high as 29.
The report found that best practice recommends that people with dementia should live in small domestic-type dwellings of no more than ten people per unit.
It found that the private sector provides the bulk of dementia-specific residential care, rather than the State.
The report calls for fairer funding for the private sector, given the complex and high dependency needs of this group.
In general, private specialist care units were more likely to be purpose-built and well-designed, offering individual bedrooms, often with en-suites, when compared with others.
The authors said a key finding is that the private sector is the main provider of specific long-term care for people with dementia, while the public sector is more likely to be the main provider of residential respite care.
Waiting lists are very long in areas with low provision.
The study involved the Dementia Services Information and Development Centre at St James's Hospital and the School of Social Work and Social Policy at TCD.