Would it have been advantagous to Ireland in terms of economy? I'll freely admit that post 22 through to the 60's Ireland's economic policies were "unwise", yet I'm doubtful that the UK would have been willing to invest heavily in the wider nation, would the UK have been willing to extend the budget supports to all Ireland that they extended to NI? Would a Westminister Government/public be happy at sending 30+ billion a year to Ireland (just assuming that NI's current transfer is tripled for all of Ireland). Would the domestic companies in Ireland have come into being or would English companies have dominated? Would Ireland have remained heavily rural/agricultral or transitioned to the high tech sectors that we now have (not too mention that we couldn't have competed with England as we do now).
In terms of Social history, considering NI is still the most socially conservative of the Union, short of an early breaking of the Catholic Church I don't see Ireland changing massively (particularly if there was some form of Home Rule), like wise I doubt Ireland would have enough MP's to majorly delay any decisions the rest of the UK made.
Yes I think Ireland would benefit economically. For one thing there would be an economies of scale effect and another no Anglo-Irish trade war.
IIRC Ireland had 100 MPs in Westminster if most of the Irish voted for an Irish political party (instead of Labour, Liberal and Conservative like Scotland and Wales did) that often held the balance of power then the Westminster Government would often have to give in to their demands for infrastructure and industrial projects (like an extension of the Transmission System of gas pipelines).
You are probably right that the Westminster Government/public would not be happy about spending +30 billion, but I think they would still do it in spite of that. It would be a continuation of the killing the independence movement through kindness policy. It's somewhat like the Westminster Government spending more money per head on Scotland than it does per head in England.
On the other hand the Greater-UK would be making a smaller net contribution to the EU. That is what it pays in would be offset by the money that Ireland gets out. Therefore the EU might be more popular/less unpopular with the English.
I simply don't know if the Southern Irish economy would have developed along the lines you outlined.
I don't know if the social history would have been different either. I think that when abortion and homosexuality were made legal in Great Britain they remained illegal in Northern Ireland. If I am correct that might have been to placate the Northern Irish MPs or it was because Northern Ireland was a self-governing province at the time. Perhaps ITTL the reforms would not have been applied to the whole of Ireland if there had been a United Ireland within the United Kingdom regardless of whether it was governed from Westminster or if there was an Irish Parliament with the same powers that the Northern Irish Parliament of 1921-72.
However, on a half-serious not the late great Dave Allen, did a routine on why there were so many Irish people in England. His answer was that they weren't here for economic reasons, they were here for the sex.
Also half-seriously I think the UK should have built the North Channel Tunnel from Scotland to Ulster instead of the English Channel Tunnel.