Paul Stirling has committed his future to Ireland and will leave Middlesex at the end of this season.
Stirling has signed a new contract with Cricket Ireland and will depart Middlesex after 10 years with the English county side.
The 29-year-old batsman has played almost 200 games and scored more than 6,000 runs for the club.
"It's been a real privilege to represent Middlesex Cricket over the last decade," Stirling said.
"It has been an unforgettable journey for many reasons. Winning the Championship in 2016 was undoubtedly the highlight on the field, while also receiving my county cap will be something I will cherish for a very long time."
Ireland's elevation to Test status in 2017 has meant that after the 2019 season, Irish internationals in English county cricket must re-register as overseas players - or give up their international careers to remain as local players.
Another Middlesex and Ireland player, seamer Tim Murtagh, is yet to announce his decision.
Stirling added: "I am looking forward to the next chapter with Ireland. With a busy year in 2020 and the T20 World Cup Qualifier coming up, it is an exciting time to be a part of."
The draw for next month's T20 World Cup Qualifier was made on Tuesday, with Ireland placed in a group with United Arab Emirates, Oman, Hong Kong, Canada, Jersey and Nigeria.
The tournament, which will be played between 18 October and 2 November in the UAE, will see six of the 14 competing teams progress to the T20 World Cup in Australia next year.
Two groups of seven teams will compete in a round robin with the top team in each group automatically qualifying for the World Cup.
Teams finishing second to fourth in each group will enter a play-off series to determine the final four qualifying teams.