Ian O'Brien and two friends even enjoyed lunch on the set at Bective, where they spent most of the day during filming.
At one stage they were discovered by security while hiding in a bush but instead of leaving, they donned face masks and yellow vests to blend in with the crew and returned and stood behind the main camera.
Ian, from nearby Dunderry described the experience as "a phenomenal day when three gobshites blagged their way onto a multi-million film set."
"I was googling Matt Damon last Monday morning and decided that by hook or by crook, I'd be heading up to Bective," he laughed.
"I know a few of the landowners so I met up with one of them Michael Lally and we walked along the Boyne on his land adjacent to the set. Sure he had to check on his cows anyway!
"I saw horses on the bridge with flags up and I knew the bridge was the focal point of the filming so I thought there might be some action there.
"Then I saw Matt Damon getting up on a horse and sure I was chuffed with myself. I'd have headed home at that point but Michael had more cattle behind the set he had to check on so security let us pass.
"We fell into the bushes and watched things unfold for a few hours . Sure we were like school kids hiding in the ditch. We stayed there until another friend Michael Keaveny came along. He was wearing a light coloured top which gave the game away.
"A security woman came over and asked us to leave but just as we were heading back to the van, we noticed that all the crew were wearing yellow vests and masks and we had them in the jeep so we put them on and walked right into the middle of the set.
"We were pure chancers!"
"For about three hours, we stood behind the main camera and marvelled at how it was all done and watching the hangings on the gallows.
"Then a lad shouted up to us - chicken or beef! Sure I said beef and the others said chicken and a little while later we were given our lunch in compostable pots.
"The lads got chicken pasta that was second to none and I got a kind of beef stroganoff. I'm shocking picky about my food but it was lovely," he laughed.
"We even got buns and cakes afterwards
"Everyone was moving down the hill at this point so we went with them and found we were standing five yards away from Matt Damon who was chatting to a few fellas. So we got a few sneaky pics and we were delighted with ourselves.
"Everyone was wearing masks and so careful about social distancing - we even got shouted at a few times to keep two meters away from each other.
"I have to say it was so well run and a phenomenal day. We fell around the place later thinking of the three gobshites from Meath who used cows to blag their way onto a multi-million euro film set."