The former New South Wales player, who qualifies for Ireland after marrying an Irish woman, has already led Ireland to a place in next year's World Cup, an achievement which has earned the island official One-day status for a two-year period that begins with the visit of England.
"When I was growing up playing cricket in Sydney it was my dream (to play in the Ashes)," Johnston said here on Tuesday.
"But things didn't work out that way, I wasn't good enough to make that. Now I am proud to play for Ireland, my adopted country.
"I am married to an Irish girl and we decided to move back two years ago and hopefully pursue a dream to play in the World Cup. The World Cup was a huge carrot and you cannot pass up that sort of opportunity."
Johnston is in the same position as his compatriot Jeremy Bray and South African Andre Botha.
Ireland also have a have South African coach, who rejects any idea that the trio are cricketing mercenaries.
"All of the foreigners live in Ireland, they have married Irish girls and have Irish kids," Birrell said. "What we haven't done is gone out and found people with Irish passports just to play in the World Cup.
"We are using players that are not going to disappear straight after the World Cup, they are all contributing in some way to Irish cricket."
With England far from full strength, Ireland may just be fancying their chances of another upset, having humiliated Zimbabwe by 10 wickets in 2003 and defeated the West Indies by six wickets at their Stormont base the following summer.
"We showed what we are capable of two years ago against West Indies, the last time we played an international team here," Johnston said.
"We gave it our best and they probably didn't turn up on the day, I suppose. Six weeks later they beat England in the Champions Trophy final. Things like that can happen in cricket and we are going to go out there, do the basics right and be competitive."
Andrew Strauss will skipper England in the absence of both Michael Vaughan and Andrew Flintoff through injury.
The Middlesex batsman, who takes command of a one-day squad that includes six players yet to make their one-day international debut, warned that there would be no room for complacency as England warm up for their upcoming five-match series against Sri Lanka.
"All games against lesser playing nations are potential banana-skins, but we've got to look at it in the context of the series we've got coming up and that means we have to get some useful practice and try and be clinical," said Strauss.
"That's something that's going to be very important against Sri Lanka - if we can get on top of them we've got to try and finish them off. This is a very useful game in that respect and we won't be taking it lightly either.
"Ireland have had a couple of surprises in the C and G Trophy and they're clearly capable of playing some good cricket.
"We'll treat it like a one-day international and if we do that then we'll give ourselves the best chance of doing well."
Middlesex batsman Ed Joyce was missing from England's training session at Stormont on Monday with a stomach bug which could prevent him making an emotional debut against his native Ireland.
Joyce, who has completed the three years qualification to play for England, could line up against brother Dominick - who is poised to feature in Ireland's line-up - if he is passed fit to make his debut.