Your undergraduate degree is very important, as the degree from law school is more of a qualifying degree. However, if you have a second class lower degree during your undergraduate studies but you have a first class or second class upper degree in law school, you should emphasize your better academic result.
In your personal statements, you can emphasize your success during law school and if applicable, you can give extenuating circumstances for your performance during your undergraduate days.
However, you must also remember that graduate school application is NOT ONLY about your grades! Your work experience, extracurricular activities and references can go a long way to convince the application committee that you are a viable candidate. Do not let your grades hold you back!