Could it have being possible for William Pitt the Younger to have given the Catholics the Emancipation he promised with the act of Union? If that had of happened perhaps the calls for repealing the act of union would not be so heavily linked with the calls for Emancipation? Maybe the Union would be seen as a good thing in Ireland since it brought with it good change? Despite the British public's opposition to emancipation, the biggest hurdle was the king. Could he have slipped into madness early and the legislation moved from there? If not, could an earlier POD have forced the acception of emancipation due to different circumstances on the ground? Another issue would be the Tithe, would that be dispenced with quicker if Emancipation occured three decades eariler?
Kvasir
Possibly but very difficult I suspect. It wasn't just the king as much of the establishment and much of the population that opposed it. An earlier attempt to give equal rights to Catholics in Britain had prompted the so called Gordon riots in the 1780's.
Also if emancipation had been achieved it would really have angered and worried the Protestants in Ireland and their supporters elsewhere. The Act of Union, contrary to later propaganda, was a good step for the Catholic population of Ireland as it achieved its purpose, of removing control of Ireland from the Protestant minority, which had been a cause of much resentment and hostility in Ireland. To a degree I think it was a compromise, in that the Protestants gave up their parliament but avoided any danger, at the time, of coming under Catholic control. You could have had a very explosive situation in Ireland, even more so that OTL if London had tried to enforce emancipation and left the Dublin Parliament in place.
Steve