In Stirling for a Better Together rally, the Labour peer responded to remarks by Swedish foreign minister Carl Bildt, who last week said a Yes would lead to the UK's "Balkanisation".
A UN special envoy to the Balkans between 1999 and 2001, when the break-up of Yugoslavia into warring states resulted in appalling ethnic violence, Bildt said the end of the Union would have profound and far-reaching implications.
"The Balkanisation of the British Isles is something we are not looking forward to. What are the implications for the Irish question? What happens in Ulster?" Bildt said.
Some commentators believe Scottish independence would give renewed impetus to the demands of Irish Nationalists for a united Ireland.
But Reid (pictured right) said the referendum did not have "a particular implication for Ireland" and that Scotland and Ireland are separate issues.
"I welcome the fact that in recent years there has being a growing rapprochement between the governments of the UK and Ireland.
"At a time when there is, in a sense, a coming together after so many years, indeed centuries, of the UK and Ireland, I think it would be paradoxical if Scotland was to separate from the rest of the UK at precisely that time."
However, he added Bildt's comments and Barack Obama's coded support for the Union last week showed the magnitude of the decision ahead.
"When the President of the United States says that, it's an indication of the difficulties. When the prime minister of Spain, the president of the European Commission and the foreign ministers of the Scandinavian countries say that, it's an indication that they are concerned about the wider implications of it."
In his speech, Reid likened the Yes camp to Monty Python's 'What have the Romans ever done for us?' sketch.
He said: "What has the UK done for us?
"Well, it gave us financial stability from the period after the Darien Scheme through ... to RBS ending up with toxic debt greater than the GDP of Scotland.
"They, say: 'Well, I give you that - but what has the UK ever done for us?'."
Alex Salmond's pursuit of independence was "blinding him to reality", he said.
SNP Health Secretary Alex Neil dismissed Reid as a "Labour failure", adding: "If we vote No, we will never be taken seriously in London again."