I will throw this out there.
The best way to a UI, imo, through exclusively peaceful and democratic means, is to revert back to the last time the country was united as one political and economic entity and there was no such thing as paramilitaries and the talk of violent insurrection was practically non-existent or extreme fringe thinking of little relevance.
I revert to 1909 when Queen Victoria was paraded through the packed streets of Dublin, of native admirers and loyal subjects in one United Ireland.
The usurpation of the democratically achieved parliament for Ireland, through threats of violence from newly formed paramilitary of Ulster Volunteers, was the catalyst of rebellion, guerrilla warfare, partition, civil war, social disorder, and the 25yr conflict.
Good riddance 20th century Ireland.
Personally I think sectarianism is the root of all this conflict. Partition has only served to perpetuate that sectarianism, on both sides of the conflict.
A UI, one political parliament, with fundamental guaranteed constitutional ties to the British monarch, as members of the EU, with constitutional protections for identity and culture, traditional flags and emblems.
An Irish national anthem - Danny Boy
Irish national flag - Harp, Shamrock and Crown.
After that, we all just get on with the normal business of managing our social and economic affairs through normal parliamentary affairs.
Whats not to like?