The word Lilly (with a double L, first letter capitalized) is a person's name; if you know two people named Lilly, they are the Lillies. The word lily (single L, all lower case) is the flower; the plural for lily is lilies.
The plural form for the noun dress is dresses, a regular plural form. A regular plural is a word that forms the plural by adding an -s or an -es to the end of the word. An irregular plural is a word that forms the plural in some other way.
Actually, the plural form radiuses is accepted as correct and has found it's way into dictionaries (see the link below). The word radius and the plural radii are derived from the Latin word radius, a word for the spoke of a wheel. The plural form is a standard form for a plural in Latin.