The Act of Union was achieved with mass bribery of Irish peers; with typical British… thrift, it was then repaid out of the taxes raised in Ireland.
It was disastrous for Ireland, because the rich migrated to the new centre of power in London, reducing custom for craftspeople, farmers, retailers, etc; landlords became absentees with profit-grinding agents representing them.
The foundation of the Irish Free State in 1922 after the War of Independence created a partitioned island - with six counties, three of them less than willing, remaining in the United Kingdom and 26 in the new Free State.
In 1937 the country (the 26 counties) was named Ireland and effectively became a republic, with an elected non-executive president as head of state. It was officially declared a republic in 1949, on the enactment of the Republic of Ireland Act.