The Clare by-election of 1828 was notable as this was the first time since the reformation that an openly Roman Catholic MP, Daniel O'Connell was elected.
Clare was held by William Vesey Fitzgerald when he became the President of the Board of Trade. As this was seen to be an office of profit, Vesey-FitzGerald had to stand in a by-election.
It was not unusual for these by-elections to be uncontested. However the Catholic Association, a group campaigning for Catholic civil rights and the repeal of the Act of Union, had vowed to oppose every member of the current government, who had declined to allow for Catholic Emancipation. A number of candidates were approached but refused to stand, Vesey-FitzGerald was reasonably popular with Catholics in Clare.
Daniel O'Connell decided to exploit a loophole in the Act of Union. It stated that Catholics could not sit in Westminster as a Member of Parliament (MP), but there was nothing about them standing for an election.
Like all Parliamentary elections at the time, Clare was held as an open vote, which meant that all votes would be known. This meant that Protestant and pro-union landowners could influence their tenants, who were far more likely to be Catholic and anti-union.
O'Connell won the by-election. Since he was a Catholic, he could not take the Oath of Supremacy, which was incompatible with Catholicism and so could not take his seat in parliament. This meant that his demand rose to allow him to become an MP for County Clare as it did not have representation. O'Connell hinted that he would get more Catholics elected to force the situation saying "they must crush us or conciliate us".
The Prime Minister, the Duke of Wellington, and the Home Secretary, Sir Robert Peel, even though they opposed Catholic participation in Parliament, saw that denying O'Connell his seat would cause outrage and could lead to another rebellion or uprising in Ireland, which was about 85% Catholic. This led directly to the Catholic Relief Act 1829.