Last night I found myself thinking on this one a lot for some reason. WI the 1800 act of union does not pass?
Let's say that those in Britain who are really not too keen on the idea voice their opinions more vocally and are able to draw in a bit of support thus the entire effort is rather more half-hearted and falls flat. Let's also hand wave away any attempts to try again for a fair few years at least.
So...how do things go for 'free Ireland'?.....
My thoughts- not well.
1: Catholic emancipation: Dream on.
In the UK this did happen. Not right away due to the king getting in the way but by 1829 it had passed. Religion just wasn't that big a deal anymore, though barring Catholics from parliament may well have been a very prudent move a century ago the threat to Britain has long since passed by the 19th century. Besides, how many Catholics actually had the wealth to be able to vote? And how big a percentage of the total population of the UK were Catholics? In both cases the number was rather small.
Ireland meanwhile was in quite a different situation to the UK. There the Catholics quite overwhelmingly outnumbered the protestants; by 9:1 a quick wiki check tells me. Even considering a wealth based franchise Catholic emancipation could really threaten the very shape of the country's government. Given that the country was ruled by the protestants its highly unlikely they would ever consider giving up their power. Purely looking at this point alone I could well see the protestants continuing to rule Ireland well into modern times when Britain and other countries finally begin yelling at them to stop being medieval gits.
2: The potato blight: It occurring exactly as it did IOTL is a bit ASBish but it does seem likely that something would happen to Ireland's potato crop if they continued with the monoculture that they did have. What would be the effects of them being out of the union if such a thing did happen though?
Again, I don't think things will be the rosy picture the Celtophiles paint.
2.1: Export/Import of food- Unless a trade deal was reached separately from a act of union (this could well happen I believe, Ireland was a very tempting market for Britain) trading restrictions would still exist between the UK and Ireland. This could well work out well for Ireland here on the surface, one of the most common arguments against the British in the famine is that Ireland exported food still during the famine. Of course those of who who know something respond that this food was privately owned and that lack of food was not the problem. In this alternate timeline without the union perhaps selling food to the mainland would not be so profitable, there was afterall the corn laws (they came into place after the POD but...the situation remained mostly similar sans Ireland) and it would have to stay local? Perhaps. Big perhaps.
This may save a few lives however the problem with the famine was that those dying just didn't have any money to buy food. More being available would help little here other than dropping the prices- they'd still be out of their reach though.
2.2: Government- As said the British get a lot of flack for the famine. The government did indeed respond quite stupidly to it, they put all their faith in their market system to right itself (much like Hoover during the depression).
What of the alternate Irish government though? This is quite unanswerable for me. It'd require quite a indepth study of Irish political parties and extrapolating trends. They could take more of a active role in trying to stop the famine or they could also rely on it to fix itself. It could go either way, better or worse.
Another thing to consider though here before we finish is that this Ireland is ruled by the protestant rich. Would they really be too bothered about poor Catholics dying en masse? Could many of them in fact perhaps welcome it? Perhaps ITTL the famine could actually be a genocide. One the British would have to step in to stop. Now that would make for a interesting TL....
2.3: Charity- A big thing that is forgotten by many in their ranting over the famine is the massive charity effort across the country to help the Irish. Huge sums of money were raised all across Britain to help the Irish poor. It was front page news, a horrible thing happening to their fellow countrymen, how could any decent Briton not throw in a few pennies to help some poor suffering paddy?
What would happen in this time line though? Would the famine remain such a big deal? It wasn't happening in the UK here, just in their poor neighbour.
Additionally, assuming all the funding is there, would British charities be free to act in Ireland the way they did IOTL? Both countries are in personal union and Ireland is suffering so on the surface it seems likely they would be welcomed but...then you have to consider the incident of Queen Victoria and the Ottoman Sultan. She'd be embarrassed if he helped more than her.
Consider that prior to the act of union the Irish government were quite fiercely independent, they really didn't like being seen as Britain's poor relation. It seems likely at the very least that charitable efforts would be severely hindered here.
2.4. Emigration- So many things to consider here.
Firstly- moving to Britain would not be so easy. They were not the same country, laws could well be put in place specifically to stop cheap Irish labour drowning out the British- personal union or no such concerns existed as early as the 17th century with Scotland and England in union.
Secondly- moving to the US. Americans were rather racist people and they were no great fans of the Irish. Could it be possible they too would hinder Irish immigration? This would be a lot easier done with Ireland as a separate entity rather than it being mixed in with the highly desirable British immigrants.
3. Economy- Britain led the way in industrialisation. Much of this seeped over into Ireland too. With a border standing in the way would it go as strong? Perhaps even Britain would actively seek to stop it so that British industry could dominate.
4. Rebellion- So, we have the big nasty Irish protestants actively stamping on the country rather than the quite indifferent British government just making sure things keep ticking over smoothly. Could this then lead to a repeat of 1798? Perhaps several of them.
The Irish government would of course call on the British to help them out and the British would have no choice but to do so but what of the aftermath? Would they leave things as they are or make changes to stop another rebellion?
5- Union- Its defeated in 1800 and immediate future attempts are stopped but what about further into the 19th century? Would the time come that Ireland eventually does join the union? Perhaps as the aftermath of 1798 2.
So there...Just organising my thoughts onto a page. Perhaps a TL to come in the future. For now though I'd be interested on others views.