The 39-year-old called time on a glittering 198-cap international career on Friday when he lifted the ICC InterContinental Cup - his 12th title in a glorious ten-year span that also saw him feature in two World Cups and three World T20s.
"It's very exciting and a great honour to be given these new challenges by Cricket Ireland," said Johnston, who takes over the women's role from his great friend Jeremy Bray. "It's certainly a new chapter in my life and one I'm looking forward to immensely."
"I'm on a two-year deal to coach the Ireland Women, and it'll be a baptism of fire as we're taking part in a tri-series in Qatar in January with Full Members South Africa and Pakistan.
"That's followed by Ireland's first T20 finals in Bangladesh. It's going to be tough because we'll be the only Associate Member there - but I wouldn't have it any other way.
"The Qatar series will be ideal preparation for Bangladesh. It'll let me get my feet under the table and get to know the squad. I've worked with most of them before but this is different in that I'll be Head Coach this time around and I'll have ultimate responsibility. I'll enjoy that and hopefully in time I'll get them playing the brand of cricket that I've played."
Johnston has also been charged with moulding the next generation of Ireland pace bowlers with his part-time appointment as National Academy Fast Bowling coach.
"It's a great job fit for me and it'll be great helping shape the future of Irish cricket," he said. "I had discussions with Richard Holdsworth and National Academy Manager Craig Hogan and they were both keen to keep me involved with this side of things, passing on my knowledge and experience.
"I'm going to be busy as I'm also keeping my role as Leinster Lightning coach. I'll be surrounded by good people and I'm certainly going to take advice from those around me - I'm sure I'll be on the phone to 'Simmo' bouncing ideas around."
Performance Director Richard Holdsworth was delighted that he was able to keep Johnston in the Cricket Ireland family.
"Trent has played in over 15 countries world-wide and just under 200 times for Ireland," Holdsworth said. "He has moved further into coaching in recent times, and we firmly believe he is the right man to lead the Ireland women's team for the next two years.
"He has an enormous understanding of the game, a work-ethic which is second to none, and is constantly willing to learn. He has excellent technical and tactical knowledge and captained Ireland 60 times.
"Trent knows how important team ethic is in winning teams and that is sure to rub-off on the players. He has worked with current head coach Jeremy Bray for some years and shared much of his knowledge about the players.
"Jeremy has worked tirelessly with the girls over the last few months preparing them for the remainder of the winter and the World T20, and I would like to personally thank him for all his efforts in helping the team achieve the success it has in the last two years - not just in ICC events but also being promoted twice in the ECB Women's County Championship."
Holdsworth is confident that Johnston will prove a perfect role model for the young Academy bowlers and the Ireland under-age squads.
"It is fitting that the man who has led the Ireland bowling attack for ten years, is now going to be instrumental in the development of the next generation of young quicks.
"Trent's knowledge of fast-bowling is extremely good - not only the technical side, but a key understanding of the work ethic required to succeed, the tactical side of bowling in three different formats, and the mental approach to this part of the game. He will inspire many young bowlers with his enthusiasm and motivation".
Johnston will commence his role at the beginning of next year, cutting short a family holiday to Australia, to take the team to Qatar in January for the ODI and T20I Series with South Africa and Pakistan.