He was 51.
Ryan was elected to Glasgow City Council in 1995 and represented the Shettleston ward.
It is understood that he suffered a heart attack yesterday afternoon.
Leading tributes, Glasgow City Council leader Gordon Matheson said: "The people of Shettleston have lost a powerful champion and the city has lost a fighter for social justice."
Ryan is survived by his wife Linda, and three children, Christopher, Deborah and Sophie.