While he will appear in The Last Jedi, his involvement in its sequel is not yet verified.
Domhnall Gleeson is excited to work with JJ Abrams again in Star Wars: Episode IX.
While The Last Jedi still has a few months to release, our attention has been temporarily diverted to the still-untitled Episode IX because of the director switch – a predicament Lucasfilm has found themselves in more than once recently.
Interestingly, Gleeson’s answer is non-committal about his involvement in Episode IX, presumably to prevent any inadvertently revealed spoilers.