The thing about negotiations is that they are that, a negotiation: you have to be willing to compromise and trade. De Valera doesn’t exactly have a reputation for compromise.
Frankly, the Long Fellow should have gone to the negotiations.
Personally, I subscribe to the view that he knew a compromise was inevitable, by that point, and wished to insulate himself from the ramifications.
It made little sense, IMO, to send Collins.
Far better to have kept him at home, with a firm hand on the tiller, as an ace in the hole (in the event of a resumption of hostilities).
That aside, the only way I see to avoid Partition would have been to avoid the Rising (and its' Aftermath).
Have the King weigh in on the side of Home Rule as a lesser evil than internecine conflict.
This could have a sobering effect upon the public discourse and could be sold to the Monarch as a way of demonstrating the way the Crown transcends politics.
That may require a stronger expression of 'Unity through the Crown' but (absent the Rising/Aftermath) could be possible.
It would also open great opportunities for ATLs - "What if...No Newgrange Intervention?"
When the Great War occurs integrate the 'Ulster' and 'Irish' Volunteers and have the shared experiences forge bonds that transcend the sectarian divides.
With no Rising, or an abortive attempt exposed by O'Neill, Ireland focuses on the shared sacrifices on the Western Front.
Post War, Home Rule is grudgingly accepted but is made to work as a pragmatic necessity.
Ireland struggles through the 20s and 30s, leaning leftwards under the blows of the Depression.
Relations with Britain are occasionally fraught but essentially solid, if only from economic necessity.
The Second World War sees Ireland standing at Britains' shoulder and benefiting greatly from an industrial boom.
Shared suffering, at home and abroad, including the Dublin and Belfast Blitzes, further cements internal cohesion.
Post WWII, Ireland develops along 'Canadian' or 'Australian' lines, active in the Commonwealth, acknowledging the Crown as a unifying symbol, but effectively independent.
This scenario sees a less 'Celtic' Ireland, industrialised to a greater extent (and consequently more Left wing), freed from the dead hand of confessional politics and Republican/Unionist ideological shibboleths.
Falkenburg