Because the law mandated that they should convert. The Catholic Church being an illegal organization until 1828, with lingering disabilities until 1920.
The Roman Catholic Relief Act 1829, passed by Parliament in 1829, was the culmination of the process of Catholic Emancipation throughout Britain. In Ireland it repealed the Test Act 1672 and the remaining Penal Laws which had been in force since the passing of the Disenfranchising Act of the Irish Parliament of 1728.
This removed many of the remaining substantial restrictions on Roman Catholics throughout the United Kingdom of Great Britain and Ireland. However, at the same time the minimum property qualification for voters was increased seriously reducing the number of those entitled to vote, although after 1832 the threshold was again lowered in successive Reform Acts.
The major beneficiaries were the Roman Catholic middle classes, who could now enter careers in the higher civil service and in the judiciary. Lower class Catholics took far longer to catch up.
During the famine a phrase evolved "he didn't take the soup" referring to people who converted to get food from food kitchens.
Later because of conversions to Protestantism the Catholic Church and a growing trend of mixed marriages, they brought in
Ne Temere. This decree was issued under Pope Pius X, 10 August 1907, and took effect on Easter 19 April 1908. A priest then could refuse to perform mixed marriages between Roman Catholics and non-Roman Catholics, he could impose conditions such as an obligation for any children to be baptised and brought up as Catholics, and for the non-Catholic partners to submit to religious education with the aim of converting them to Catholicism.
You can see in some 1901 census returns where the family are shown as both religions and the sons the as the father and the daughters the same as the mother!
