In this article, the author proposed the hypothesis that the nuclear family based on the nuclear family system was predominant in early nineteenth century Ireland. However, the stem family must also have existed. The author assumed that such stem families were supported by family conditions and could be explained by a life cycle in the nuclear family system. We divided the subjects of survey ground into 3 areas in consideration of economic regionality, namely regarded Meath and King's as the first region, Cavan and Fermanagh as the second region, and Galway as the third region and has verified the proposed hypothesis based on the records of the 1821 census returns. In Meath and King's, it can be said that the facts that the compound family households was predominant among large landholding farmers and that the simple family households was predominant among laborers reflected the family strategies behind each household type. In Cavan and Fermanagh, people adopted the family strategy of manufacturing linen as weavers and spinners. This family strategy among small landholding farmers included early marriage based on the land division system, leading to the predominance of the simple family households. In Galway, the householders in this poor region had their children wait for inheritance so that the children would take care of them in their old age, and these children acted according to this family strategy. This is why there were an unexpectedly large number of compound family households. However, the forms of these households corresponded to their life cycles. Based on the aforementioned analysis, it was found that the nuclear family system was predominant in early nineteenth century Ireland. However in Meath, a region featuring large farmers, the conditions to shift to the stem family system have already existed in the midnineteenth century. Such an understanding allows to position changes in the family structure of Ireland as a smooth process to shift from the nuclear family system to the stem family system.