The Hertfordshire outfit secured a spot in the last four for the first time since 2016, where they ironically lost to their south London opponents.
The Harrow-born defender did not play in that particular game, although he featured in the subsequent final, where Palace lost to Manchester United.
While thoughts of supporters turn to what may be the first of two Wembley visits, Watford still have a very realistic chance of making the top seven.
For Mariappa, both cup success and an impressive league position is down to hard work.
“It’s just business as usual, we’ll keep training and keep trying to do what we’ve been doing up to this point in the season, and in every game try and get as many points as we can,” he said.
“There’s a lot of football still to be played this season, we’re buzzing but there were no flying champagne corks.
“When it comes to the semi-final we’ll take it from there.
“We’ve enjoyed the cups this season, as a squad, and we’ve tried to put everything into it.
“Today was a real team effort, and we’re delighted to go through. We’re just enjoying every game.”
The game also saw what is likely to be one of the final appearances of Heurelho Gomes in a Watford shirt.
The Brazilian announced his intention beforehand to retire at the end of this season, having featured in all FA Cup ties this campaign.
Mariappa was quick to pay tribute to the 38-year old goalkeeper, and also praised the contribution of substitute Andre Gray, whose goal proved to be decisive.
“Gomes is hugely popular in the dressing room and there’s a tremendous amount of respect for him, you can see he’s loved by everyone from club to fans. I’m hoping it’s not his last game here,” he added.
“We’re always confident in him, whatever game he’s played in. I don’t think anyone has ever doubted his ability.
“I was delighted for Andre to come on today, and he’s a great attitude when he hasn’t been playing. Our squad is very balanced, you can call on people from the bench.”
Palace were weakened by the absence of Wilfried Zaha before the game, due to a hamstring, and looked slightly out of sorts in the first half.
Watford took a deserved lead through Etienne Capoue, although visiting goalkeeper Vicente Guaita had been looking vulnerable on crosses from corners.
The second half proved to be a different matter, with Michy Batshuayi equalising after dispossessing Mariappa on the left hand side.
The final word fell to the hosts though, with Gray slotting home just two minutes after entering the field as substitute - a goal that seemed to demoralise Palace.
With hopes of their first semi-final since 2016 extinguished – and manager Roy Hodgson again failing to reach the last four in the competition – the visitors must now focus on Premier League survival.
It was not difficult to sense the deflation of James McArthur afterwards though, who felt the absence of the talisman Zaha may have had an effect.
“It might have given Watford a boost, if we’re sitting in there and we’re seeing the opposition’s best players are missing then you get that,” he said.
“It’s pretty devastating to be honest, I can’t put it into words that we’ve come so close and it’s really disappointing,
“We need to get going again, we’ve got a break now that we need to take, hopefully everyone can come back from international duty and everyone is fit and ready to kick on.”
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