Published data on aspects of domestic pet demographics are available in many countries. Several of these studies have linked household demographics, such as the presence of children in the household, to pet ownership. There is very little published information about the demography of domestic pets on the island of Ireland (the Republic of Ireland, Northern Ireland). This study was conducted to describe the demography of the pet dog and cat populations on the island of Ireland and to identify human factors influencing pet ownership. A questionnaire was designed and administered to households to collect data about the demographics of households and their dogs and cats. The questions related to location, building structure, social class, nationality and family structure of the household, and the sex, age and source of each pet dog and/or cat. The survey was administered by a commercial company, using computer-assisted telephone interview techniques to 1250 households selected using random digit dialling and quota controls. In this study, a pet dog was defined as a dog that was been fed by a household and considered a pet by the participant of the study. A pet cat was defined as a cat that was both fed by the household and allowed into the house. The results show that 35.6% of households in Ireland have one or more pet dogs and 10.4% of households have one or more pet cats. In total, 47.3% of pet dogs and 76.1% of pet cats were neutered. Females of both species are more likely to be neutered than males. Factors associated with dog ownership included location, house type, household social class, household composition, the presence of school children in the house, and the presence of a cat in the house. Factors associated with pet cat ownership included the type of house structure, the presence of a dog in the house and the gender and age of the participant. Cats tend to stray into households. This study was the first to provide detailed information about the demographics of the pet dog and cat populations on the island of Ireland, and has identified areas for further research, in particular the effect of stray dogs and cats on the owned pet dog and cat population, and of future demographic trends in these populations.