There were four Irish counties where a major interest, either active or dormant, was present: the FitzGerald interest in Kildare, the Butler interest in Kilkenny, the Fitzwilliam interest in Wicklow and the fragmented Boyle interest in Cork. Of these the most active was the FitzGerald interest in Kildare, where the Earls of Kildare, later the Dukes of Leinster (0734, 0745), owned about a fifth of the county and possibly more at the beginning of the century. In the 1690s the county was represented by George FitzGerald and his father Robert FitzGerald, the son of the 16th Earl of Kildare. In the middle of the century there was a dearth of available direct members of the FitzGerald family, but this was remedied by the very large family of the 1st Duke and Duchess of Leinster, although their sons were not qualified until the 1770s. At the end of the century the Duke, who came from a large family and had a large family, was financially embarrassed and the wealthy La Touche banking family bought estates in the county. Co. Kildare's proximity to Dublin gave it many of the social characteristics of Co. Dublin, and it was dominated by the social pre-eminence of the Duke, whose principal residence, Carton, was near Maynooth. In a society intensely concerned with hierarchy, this was an important element. In 1785 the county was said to be 'totally under the Duke of Leinster's influence',194 and had he been more intelligent he would undoubtedly have been politically very powerful. It was said that 'With all his estate and all his rank, and all his good nature, his mind is so weak and his conduct has been for more than twenty years so absurd, that he is very little formidable. But this small trait proves that if anything like a consistent opposition arises his weight will be thrown in to the scale.'195
The electorate was small: it was said that in the 1790s, despite his vast estates, the Duke did not have more than a hundred registered freeholders, and between 1790 and 1797 it was said to have declined from 1,500 to 300;196 even in 1815 the number of voters was thought to be about 2,000.197 The status quo was also maintained by the fact that both the Duke and the La Touches, with whom he shared the county interest in the early nineteenth century, were supporters of the Catholic interest.
Co. Kildare had four parliamentary boroughs - Athy, Harristown, Kildare and Naas - all of which were disfranchised at the Union.