Not sure where you heard this, but it has to be flawed. With the amount of guinness sold, it would be incredibly expensive to use malt extract, especially hopped malt extract, given that they probably do use hop extract. And according to guinness themselves "All the GUINNESS® sold in the UK, Ireland and North America is brewed in Ireland at the historic St. James’s Gate Brewery in Dublin."
And how do you figure that they haven't been Irish for long, "1759 -
Arthur Guinness signs a 9,000 year lease on a disused brewery at St. James’s Gate, Dublin for an initial £100 and an annual rent of £45." 250 years seems like a while to me.