The unionists have the most to lose if the Good Friday Agreement goes forward, because they basically enjoy their goals under the status quo, and any move to interrogate the status quo or negotiate a more equitable distribution of power can only lead to a decrease in the legitimacy of their position.
The entire island had been a British colony prior to the establishment of the Irish Free State in 1922 (I'll skip the treaty and civil war bits), at which time Ireland was partitioned into the 26 counties that are currently in the Republic of Ireland and the 6 counties that Britain still maintains colonial control over.
Over the years, Britain has given up nearly all of its colonial possessions, from India (1947) to Jamaica (1962) and most recently Hong Kong (1997). Most of its remaining fruits of imperialism consist of tiny islands more symbolic than anything else (see Falklands). Yet it stubbornly maintains this contentious piece of real estate on the island of Ireland.
Perhaps with continued IRA overtures and political developments such as devolution in Scotland (even some Welsh with Plaid Cymru are reasserting at least a consciousness of independence), the unionists will have no choice but to respond seriously to the Good Friday Agreement.
However, it's important to note that nationalism will only get you so far. As James Connolly said back in 1897,
If you remove the English army to-morrow and hoist the green flag over Dublin
Castle, unless you set about the organisation of the Socialist Republic your
efforts would be in vain...Nationalism without Socialism – without a
reorganisation of society on the basis of a broader and more developed form of
that common property which underlay the social structure of Ancient Erin - is
only national recreancy.
Special note to idiots out there: just because Adolf Hitler's party called themselves "National Socialists" (Nazis), it doesn't mean they were socialists any more than the former East Germany calling itself the German Democratic Republic made it either democratic or a republic.
Yes it's all more complicated than one post can encompass. It's difficult work untangling centuries of oppression and over three decades of armed conflict. As the gossip monger Drudge would say, "Developing..."