In that era, Sinn Fein split into two clear factions - the pro and anti-Treaty elements which simply could not come to an accommodation on the Treaty terms which partitioned Ireland.
The end result was a bloody Irish civil war between the anti-Treaty IRA and the pro-Treaty Free State forces which saw more IRA members executed by the Free Staters than were killed by the Black and Tans in the previous War of Independence against the British.
More than a century later, that post-civil war bitterness is still to be found in Southern Irish politics. While the civil war spawned the big two Dublin establishment parties, Fianna Fáil and Fine Gael, these bitter rivals came together after the last Dail General Election to keep Sinn Fein out of power in Leinster House.
As for the DUP in 2024, party leader Sir Jeffrey Donaldson finds himself in yet another Yes/No confrontation over the current deal as he did in 1998 in the UUP during the Yes/No confrontation over the Good Friday Agreement.
While the UUP went into the Stormont Assembly in 1998, the political seeds were sown in the party which ultimately led to both the leader, the late David Trimble, and the party’s demise. The UUP lost its place as the lead party in Unionism to the rival DUP in the 2003 Assembly poll.
Put bluntly, Trimble personally and the UUP politically paid a heavy price for endorsing the Good Friday Agreement in spite of effective devolution returning to Northern Ireland for the first time since the original Stormont Parliament was prorogued in 1972.
The key question which the pro-deal supporters in the DUP must answer - what price will the party have to pay if it decides to accept what the Government has offered, in spite of the Irish Sea border still in place?
In 1998, in spite of a split party, Trimble put the best interests of Northern Ireland first and kick-started devolution. That split sparked a very significant realignment in Unionism.
While the majority of the UUP MLAs in 1998 were in the Yes camp, the Unionist family’s No camp included UUP dissidents, the DUP and new Stormont movements such as the United Kingdom Unionist Party and the United Unionist Assembly Party.
The Yes/No divide in Unionism over the Belfast Agreement also bled into the Protestant religious denominations. Generally speaking, many in the mainstream denominations - the Church of Ireland, Irish Methodism and the Presbyterian Church - backed the Agreement.
However, in hardline evangelicalism and fundamentalism, there was a move to rally a number of the smaller denominations behind the No camp.
Fronting this was a movement launched in late 1998 called the Caleb Foundation, named after the Biblical Old Testament Israelite spy, Caleb.
While the Caleb Foundation was initially an attempt to rally fundamentalist opposition to the Evangelical Prayer Breakfast movement, the participation in Caleb of leading members of the Independent Orange Order gave the perception that Caleb was nothing more than the No camp at prayer.
For example, Caleb’s first chairman was the late George Dawson, then in 1998 the Grand Master of the Independent Orange Institution, and later an East Antrim DUP MLA at the time of his death in 2007.
Practically, can the DUP accept the current deal, kickstart the power-sharing Stormont Executive and ride out the realignment storm within Unionism.
Pro-deal Unionism does not have a good track record historically. As well as the demise of the UUP in the years after the Good Friday Agreement, pro-Sunningdale agreement Unionism was comprehensively defeated in the two Westminster General Elections in 1974.
Two years earlier, liberal Unionism under Northern Ireland PM’s Terence O’Neill and James Chichester-Clark could not save the original Stormont Parliament.
The unpalatable reality which Unionism must now face is that it is already split. All that needs to be decided is who will win the realignment battle and how bitter that fight will be.
In voting terms, could contests between pro- and anti-deal Unionist candidates at the next Westminster General Election cost Unionism seats, with traditionally Unionist constituencies returning either Sinn Fein or Alliance MPs on a split pro-Union vote?
In reality, pro-deal Unionism must starting preparing for a Plan B after the latest so-called Ulster Says No campaign inevitably fizzles out. Historically, in spite of the so-called monster rallies at Belfast City Hall in 1985 and 1986 and across Northern Ireland, the Ulster Says No and Ulster Still Says No campaigns did not halt the workings of the Anglo-Irish Agreement or the Dublin-run Maryfield Secretariat near Belfast.
Remember the Union flag protests? A lot of marching and stomping about as in 1985/86, but to no avail.
Pro-deal Unionism must be prepared to face down the No camp, whether that be at the ballot box, in the Assembly (if there is one!), in the council chambers, and even in the church pews.
Pro-deal Unionism must come up with a constructive ideological way forward for the pro-Union community in Northern Ireland.
It has one small advantage over the No camp. The latter will always be ‘agin’ anything and everything; they will moan, huff and puff, but ask them what they are prepared to offer in terms of something constructive - and the silence will be deafening!
Then again, has pro-deal Unionism the political courage to face down the No camp splitters, or worse still, like the late Sixties and early Seventies when the Paisleyites were lambasting O’Neillite Unionists, the latter withdrew from politics completely.
The last thing pro-deal Unionism needs is its supporters and thinkers walking away from the political arena, leaving the field open to the Pan Nationalist Front of Dublin, the SDLP, Sinn Fein and Alliance.
Follow Dr John Coulter on Twitter @JohnAHCoulter
Listen to commentator Dr John Coulter’s programme, Call In Coulter, every Saturday morning around 10.15 am on Belfast’s Christian radio station, Sunshine 1049 FM. Listen online.