Conolly also improved his fortunes with his marriage in 1694 to Katherine Conyngham, the daughter of a prominent Williamite general, which brought with it a substantial dowry of £2,300. Conolly soon owned land in eight Irish counties along with estates in Wales. His properties brought him an annual rental income of £25,000, a massive sum at the time, equivalent to many millions in today’s terms.
Supported by his new wealth and his wife Katherine, William Conolly embarked on a political career, and was duly elected to the Irish Parliament for Donegal in 1692. He became a famous parliamentarian, and achieved the rank of Speaker in the Irish Parliament from 1715–29, a role that became so synonymous with him that he was known as William ‘Speaker’ Conolly.
In 1707 William Conolly purchased his lands at Castletown. Construction of the mansion began in 1722 and the main part of the construction was completed by 1729, although the grand staircase was not installed unit 1759–60. Its design was influenced by the renowned Italian architect Alessandro Galilei, who met Conolly while visiting Ireland. The work was directed by the noted Irish architect Edward Lovett Pearce.
Unfortunately William Conolly did not have time to enjoy his beautiful house as he died in 1729. His widow Katherine continued to live in the house and commissioned a number of spectacular follies (like the nearby Wonderful Barn and Conolly Folly) to keep local people employed during periods of hardship.