Look shareholder means she put up some capital. Which as a new company, they need. Might be a lot, might be tiny but it will reflect how much she gets should the company turn profit.
I personally would be shocked if she put up 1000s. They could have 500 shareholders. Quick look at their Instagram, he's using his UK high profile as investors but also getting irish new signs to "invest" which is...odd
This is more a PR thing, to use her name which is recognised in media, and followers to get publicity for the agency in Ireland. You can see by the way they've hired James kavanagh, Holly carpenter etc they're going in hard.
Also shareholders don't normally have a say in day to day running. Honestly they'd be fools to use grace as a sounding board for anything successful or professional