The act was set up to keep both the Catholics and Protestants
happy.
For example, the Catholics (nationalists) wanted to be under
their own government throughout the whole of Ireland, but this
meant that they could have the majority of Ireland, and be under
their own government, rather than be under a British government.
(so they were happy, that they weren't British).
And the Protestants (unionists) wanted to stay in Ireland and be
under a British government, which this act made them, so that they
could have all the benifits of being in Britain.