"They were like, 'we're too old to bridesmaids'", Megan told Radio 1 Newsbeat.
"They were flattered but they just thought it was ridiculous."
However, the pair agreed to their granddaughter's wish and in October they left their homes for the first social even in more than eight months.
"When we realised we would have to cut it down to only 15 guests altogether, and I realised I didn't even have space for my best friend, my maid of honour, it just didn't make sense."
"If they're going to come, we wanted to make it as special as we could for them," Kennedy said.
Although the grandmothers did not wear matching dresses, they did have bouquets and walked down the aisle together.
Photos of the trio were posted on social media and captured the hearts of many people worldwide.
"If we tell them they've got 5000 likes on a wedding blog, they're just like, 'who are the people liking it?'
"They have no concept of how many people have seen their picture."
The Belfast Telegraph printed the story, which Megan thinks might make Gillian and Mary a little more excited than the Instagram likes.
She sends her grandmothers copies of the paper.
"They think the whole thing's crazy, but it's been such a nice boost for them."
The newly weds plan to have a big wedding reception at the end of this year for their friends and family to celebrate as one.
