Was the Irish Catholic Confederation the closest Ireland came to a united Irish nation before 1922?
Ireland was in no sense a united nation during the Confederate years, 1642–49. (It was not united in 1922 either, but that's another story.) I think, however, that the Confederation marks the initial coalescing of what evolved into a self-consciously Irish nation over the next couple of hundred years.
The Confederation represented the coming together of elements from the two formerly antagonistic communities in Ireland, the native or "Gaelic" Irish and the "Old English" (whose families who could trace their roots back to the Anglo-Norman settlers of the late 12th century). Following the Reformation, both communities continued to adhere to Catholicism, which put them in direct conflict with the assertively Protestant English state.
Tudor and Stuart monarchs had "planted" Scots English Protestant settlers on lands confiscated from Gaelic Irish and Old English landowners. The most extensive plantations were in Ulster, home to the O'Neills and their allies, whose leaders had fled to the continent after the defeat at Kinsale in 1601. In 1641, a group of Ulster gentry planned a coup d'etat, which, although it didn't work out as intended, sparked a broader rebellion. The rebellion left much of Ireland outside Dublin in Catholic hands. It also led to the brutal massacre of large numbers of Protestant settlers (including women and children), mainly in Ulster.
Keeping track of who was on what side in the years of the Confederacy is a nightmare. While it was the first time the Old English and Gaelic Irish came close to forming a united front there was still mistrust between them, and King Charles played them off against each other in the early part of the conflict. He was more than a little busy in England with the Parliamentarians and wanted the Confederates at the very least stalled so he could pull more Royalist troops out of Ireland, or preferably split, so he could get the Old English back onside without totally caving in to Gaelic Irish demands over land.
In the end the peace deal between the Confederate council and the Duke of Ormonde in 1646 did just that, the mainly Old English leadership had made too many concessions to the King over religious freedom, the Papal envoy Rinuccini excommunicating some and backing the O'Neills.
What finally got the Confederates all playing on the same side, and with the Royalists too, was the arrival of Cromwell. It was hang together or the New Model Army would hang them all separately, though the NMA were pretty keen on firing squads too.
If by some miracle they had managed to turf the well-funded and well equipped (and in some cases downright fanatical) NMA out of Ireland or at least beaten them to a draw and achieved home rule fear of the Commonwealth might have kept that unity going for a few years. But the Old English desire to keep their perks and position vis a vis the Gaelic majority would have continued to be a source of division.