The seven-year-old clinched victory in a thrilling climax to the Classic last September, with Niall McCullagh getting Royal Diamond home in front by a head for then-trainer Tommy Carmody.
Murtagh, who was a major of the operation before taking over the licence at his Curragh yard, watched on from Doncaster 12 months ago but will this time be on board and admits it will be even more special if Royal Diamond can strike again.
“He’s in good form and came out of the last race very well. I’m very pleased with the way he’s progressing since that and hopefully he’ll be there to defend his crown,” Murtagh told Irishhorse.tv.
“The rain I don’t think will do him any harm. He won it last year on good to yielding and I don’t think it’s going to be soft.
“I think it is (a better race than last year). Everybody is coming into the race in much better shape, it’s a much tougher race and there are better horses in it.
“But, I have to say, I’m probably more happy with this lad this year than I was coming last year.
“Andrew (Tinkler, owner) is coming over to watch the race. We were all in Doncaster last year watching on TV, so I think it would be even more special if he can go and do it again.”
Royal Diamond was only confirmed for this race midweek, as connections considered missing the race to instead head to Australia for next month’s Caulfield Cup.
A trip Down Under for the Melbourne Cup in November remains very much on the agenda, however.
Murtagh said: “We were just kind of deciding which would be the best prep for the Melbourne Cup. Andrew looked at the stats and said horses that run in the Caulfield Cup or run in Australia before the Melbourne Cup generally run well.
“There was lots of things for it and lots of things against it. We got together and decided we’d give him another shot at the Irish St Leger. To do it two years in a row would be great.
“It was a big decision and I hope we’ve made the right one now.
“If all goes well he’ll be going to Melbourne. He’s got two weeks of quarantine before he leaves, he’s got two weeks quarantine out there and then he’s got nine days before the race.
“He’s a horse I think could run very well in the Melbourne Cup. He’s good from the gates, he’s got tactical speed, which is what you need, and he stays the trip well.”
Royal Diamond features in a field of 11 runners, with last year's close-up third Brown Panther the one significant defector after running a temperature and failing to eat on Thursday.
The field could yet be reduced further, with plans for David Wachman’s stable star Galileo Rock still very much up in the air.
The three-year-old’s preferred target is the St Leger at Doncaster, but he could yet run on home soil if the trainer is not happy with conditions on Town Moor.
Mick Halford’s Certerach makes his first appearance since being narrowly beaten in a Group 3 at Meydan in early March.
“He had a little setback when he was in Dubai and it is a big ask for him first time back, but there aren’t a whole lot of opportunities for him,” said the trainer.
“It should be beautiful ground and I’m looking forward to getting him started.”
Last year’s Goodwood Cup hero Saddler’s Rock has been disappointing since returning from a winter stint in Dubai, finishing down the field in the Gold Cup at Royal Ascot and when defending his Goodwood crown.
Oxx, whose stable has suffered with a virus for much of the season, hopes for an improved performance this weekend.
The trainer said: “He has run very flat for his last two runs and we are just hoping he can show a bit more spark.
“His race record this year doesn’t look great, but he seems perfect at home.
“It’s just when he’s gone racing he hasn’t shown his best because of the problem we’ve had with all the horses in the yard. We decided to stay at home for his next run and he’s just going up the road on Sunday, so I’ll be very disappointed if he runs as flat again.”
Dermot Weld runs Pale Mimosa and Voleuse De Coeurs, while Aidan O’Brien is represented by Chamonix and Ernest Hemingway. The field is completed by Ed Dunlop's Red Cadeaux, Godolphin’s Ahzeemah and Euphrasia from John Murphy’s stable.