remove the zest of 2 or more of the lemons, using a fine grater to get 2 tablespoons of zest. Squeeze out and strain the juice of these and the other lemons to get 3/4 cup of fresh lemon juice.
Pour just enough water into a double boiler pan so the water level is right below the bottom of the mixing bowl when it is sitting in the pan. Separate the eggs, putting yolks in the large bowl of the double boiler, and the whites in another stainless steel bowl.
To make the base for the tiramisu: Heat the water in the pan to a steady simmer. Remove the top bowl from heat and beat the egg yolks with 1/4 cup of the sugar and 1/2 cup of the limoncello until well blended. Set the bowl over the simmering water, and whisk constantly, frequently scraping the whisk around the sides and bottom of the bowl, as the egg mixture expands and heats into a frothy sponge, 5 minutes or longer. When the sponge has thickened enough to form a ribbon when it drops on the surface, take the bowl off the double boiler pan and let it cool.
Meanwhile, pour the remaining cup of limoncello, and all of the lemon juice, the 1 cup of water, and 1/2 cup of the sugar in a saucepan. Bring to a boil, stirring to dissolve the sugar, and cook for 5 minutes, evaporating the alcohol. Let the syrup cool completely.
In another large bowl, stir the mascarpone with a wooden spoon to soften it, then drop in the grated lemon zest and beat with a whisk until light and creamy. In another bowl, whip the egg whites with the remaining 1/4 cup sugar until it holds moderately firm peaks.
When the cooked limoncello zabaglione is cooled, scrape about a third of it over the mascarpone and fold it in with a large rubber spatula. Fold in the rest of the zabaglione in two or three additions. Now fold in the whipped egg whites in several additions, until the limoncello mascarpone cream is light and evenly blended.
Pour some of the cooled syrup into a pan, no deeper than 1/4 inch, to moisten the lady fingers. One at a time, roll a ladyfinger in the syrup and quickly place in a 9 by 13 inch Pyrex pan. Wet each cookie briefly - if it soaks up too much syrup, it will fall apart. Arrange the moistened ladyfingers in neat, tight rows in the fan, filling the bottom of the pan completely. You should be able to fit in about 20 ladyfingers in a single layer.
Scoop half of the limoncello mascarpone cream onto the ladyfingers, and smooth it to fill the pan and cover the. Dip and arrange a second layer of the lady fingers in the pan, and cover it completely with the remainder of the cream.
Smooth the cream with the spatula, and seal the tiramisu airtight in plastic wrap. Before serving, refrigerate for 6 hours or up to 2 days, or put in the freezer for 2 hours.