In a small saucepan, bring sugar and water to a boil. Cook and stir until sugar is dissolved. Remove from the heat. Stir in limoncello; set aside.
For lemon curd, in another saucepan, combine sugar and cornstarch. Stir in water until smooth. Bring to a boil; cook and stir until thickened, about 1 minute. Remove from the heat.
Stir a small amount of hot mixture into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from the heat. Stir in butter. Gently stir in lemon juice and zest. Cool to room temperature without stirring.
In a large bowl, beat cream until it begins to thicken. Add sugar; beat until stiff peaks form. Fold mascarpone cheese and whipped cream into lemon curd.
Arrange a third of the ladyfingers on the bottom of a 9-in. springform pan. Drizzle with a third of the syrup; spread with a third of the filling. Repeat layers twice. Cover and refrigerate overnight. Carefully run a knife around edge of pan to loosen. Remove side of pan. Sprinkle with crumbled cookies and candied lemon peel.