Mr Kenny will step down as party leader effective from midnight but will continue as acting leader and Taoiseach until a replacement is elected by June 2.
“Enda Kenny will be remembered in Northern Ireland for showing a true commitment to the peace process, as well as helping seek solutions to several crises which took place in his time in office,” said Mrs Long.
“He also displayed a proactive approach to Brexit and realised the damaging impact it will have on everyone on the island, whether north or south, and established a forum to help seek solutions to it.
“Although we might not have always shared the same political views on some issues, I wish him well in his future and thank him for his work as Taoiseach.”