By Padraic Halpin
DUBLIN (Reuters) - A victory by pro-independence parties in Scottish elections is significant for the campaign for Irish unity and underlines that politics in the United Kingdom is undergoing fundamental change, the leader of Irish nationalists Sinn Fein said on Saturday.
The Scottish National Party (SNP), which has vowed to hold an independence referendum if it returns to power, was headed for victory on Saturday in Scotland's parliament election but without an outright majority.
Sinn Fein leader Mary Lou McDonald said her party, which jointly leads the devolved government in British-run Northern Ireland and is the main opposition party in Ireland, will be watching what happens next in Scotland "very closely".
"The result is significant... and I think it just echoes again that politics right across Britain and indeed in Ireland is undergoing a period of fundamental change," McDonald told Reuters in an interview.
"There will be a border poll (on Irish unity), be in no doubt. We're down to a question of timing now. (But) of course a referendum in Scotland, of course independence in Scotland changes the constitutional arrangements fundamentally across Britain and will have a very strong effect here in Ireland."
British Prime Minister Boris Johnson has said he would rebuff calls for an independence vote because Scots backed staying in the UK in 2014.
Sinn Fein has been calling for Northern Ireland to unite with the Republic of Ireland for years and increasingly so since the UK voted to withdraw from the European Union in 2016 despite a majority in Northern Ireland voting to remain.
Under a 1998 peace deal that largely ended three decades of violence between Catholic nationalists seeking Irish unity and Protestant unionists wanting to remain in the UK, London can call a referendum if a 'yes' majority looks likely.
While most polls suggest a Northern Ireland referendum on leaving the UK would fail were it called today, higher birth rates among Catholic Irish nationalists may change the balance of power within a generation.
McDonald, whose party has been pushing the Irish government to prepare now to avoid the post-referendum wrangling Britain went through, thinks a border poll could happen within the next five years or at the latest by the end of the decade.
With a recent opinion poll suggesting Sinn Fein could for the first time become the largest party in the Northern Irish parliament, McDonald said elections there next year could be "hugely" significant for the future of the UK.
"Yes, there is chance that Sinn Fein will emerge as the largest party and therefore occupying the position of First Minister. In and of itself, that signposts again the reality of change and impending constitutional change."
(Reporting by Padraic Halpin; Editing by Christina Fincher)