Ireland wasn't independent before the signing of the Act of Union. It had been under effective British control for at least 120 years, since the Battle of the Boyne. The 'Irish' politicians who signed the Act of Union were members of the group known as the 'Ascendancy' - Protestant landowners and aristocrats of English descent. Although the members of the Ascendancy did to some degree see themselves as 'Irish', they didn't see any future for an Ireland truly independent from London, and they were indifferent or hostile to institutions such as the Catholic Church or the Irish language.

The Act of Union didn't change the big picture - it simply tidied up the legal situation by which Ireland had previously been quasi-autonomous, with its own Parliament, government and laws, and integrated it fully into the English state. Most of the 'Irish' politicians who signed the Act of Union were able to transfer their prominence in the quasi-autonomous Irish government to comparable roles in the English government, so they had no real disincentive to sign it, and every incentive, not least in greater security for the status quo that preserved their land and privileges.