The Roslin-born actor and writer was living in London, where he was in the throes of producing his low-budget road movie, The Inheritance, when he started to wonder how England and Scotland had come to be part of something called Great Britain. When he started to look into the events leading up to the Act -which may or may not be done away with following the forthcoming independence referendum - Barrow was amazed at what he found.
"It was so dramatic," he declares. "It was way more fascinating and complex than I would have thought. There were all these amazing characters and corruption and intrigue, in this fast-moving political sphere where all these figures had suddenly come to prominence before falling. You had people like Queen Anne, who was this ageing woman who didn't have an heir, despite having about 17 pregnancies. You had Daniel Defoe, who at the time was working in Edinburgh as a spy for the English government before he really came to prominence as a novelist.
"Then you had Allan Ramsay, who was this incredible Scottish poet and literary entrepreneur, who was setting up magazines and collecting people together. You read his poetry now, and he's a major figure. He has all this incredible lyricism and zest in the same way Burns or Hugh MacDiarmid have, and even though he comes from a working-class background rather than having had a classical education, he seems to be able to marry the classical world with contemporary life really well."
The result of Barrow's findings is Union, a new historical romp through the events behind the Act of Union, which opens at the Royal Lyceum Theatre in Edinburgh this week.
The play's timing may look opportunistic, but Barrow actually completed the first draft of Union back in 2008, and only showed it to the Royal Lyceum's artistic director Mark Thomson when he was in Edinburgh to promote his second film, The Space Between, a couple of years later.
At that time, a minority SNP government was in office in Holyrood, and any kind of referendum on independence looked like a remote prospect. A performed reading of Union was held at the end of 2011, with the play subsequently programmed. By that time, the Scottish Parliament election held earlier in the year had given the SNP an overall majority, and a referendum was suddenly looking like a very real possibility.
"The play suddenly started looking very topical," was how Barrow saw it, although "I thought that if any major theatre in Scotland was going to do something like this, they probably weren't going to invest in a brand new play by me, but would go for something by something by someone who's better known."
As it turns out, thus far at least, Union is one of few plays that are in any way pertinent to the referendum to be produced this year.
Indeed, history-based state-of-the-nation epics that speak to contemporary audiences have been few and far between in Scotland. While in England the post-1968 generation of playwrights such as Howard Brenton, Howard Barker, David Edgar and David Hare have all in different ways reimagined real events, in Scotland writers have arguably done things differently.
In the 1980s, Liz Lochhead's Mary Queen of Scots Got Her Head Chopped Off took a lyrical approach to the relationship between Elizabeth 1 and Mary Stuart, while Jo Clifford's early plays, Losing Venice and Lucy's Play, used historical backdrops to make serious points. The nearest equivalent to Barker's Victory, Edgar's Maydays, Brenton's The Romans In Britain or Hare's Plenty came in 1999 with David Greig's look at Scots pioneer of paper money John Law in The Speculator.
Alistair Beaton's Anthony Neilson-directed Caledonia, a satirical look at the doomed Darien expedition, was less successful in its execution, though it was still possible to recognise its roots in John McGrath's The Cheviot, The Stag, And The Black, Black Oil. Such irreverent but politically charged looks at Scotland's nation state arguably date back to Sir David Lyndsay's Ane Satyre Of The Thrie Estaites, while Rona Munro's forthcoming trilogy for this year's Edinburgh International Festival, The James Plays, is likely to take a more serious approach.
"I love Howard Barker," Barrow says. "To set his plays in a historical context, but to have contemporary ideas and to use contemporary language, and to say a great deal about politics is amazing. I remember seeing Barker's play, Victory, at the Lyceum, which had a massive effect on me. It made me see that something like that was possible, something that said tons about the world I live in, even though it was set in the past.
"I really admire all these writers that came through in the 1960s and 1970s, and who could have 30 actors or something onstage," he explains. "These brilliant dramatists seem to find these moments of upheaval, revolution and new ideas coming forth, and I guess in our age there's so much seeming freedom of information and ideas that we're used to the fact that anything is possible. So it's good to be reminded of certain historical situations where people are realising for the first time that they can live in a completely different way."
While Union is not anything as simplistic as a pro-independence polemic, it is clear where Barrow's sympathies lie. Yet, rather than create something dryly educational, Barrow cites Monty Python as an influence as much as Barker.
"So much of what happened is seemingly absurd," Barrow says. "So the play's not cynical, and it's certainly not any kind of biting satire.
"We're very much trying to celebrate the life, the times and the spirit of the age, and also the political arguments of the time. It's not about saying one argument's right and one's wrong. It's more complex than that. So many of the arguments for union as well as those against were absolutely sound for the time. It's only later that you discover that various people received money for voting a certain way and betrayed the people they were with. That's where it gets really difficult."
Union, Royal Lyceum Theatre, Edinburgh, March 20-April 12 www.lyceum.org.uk