Anyone who does not have the right to reside in Portugal, such as non-EU Citizens, and even those who may reside here legally - such as EU citizens - may consider it preferable to make an application for citizenship.
Citizenship confers the right on the holder to reside and work – not just in Portugal - but in any EU country. Also, citizens have the right to vote as well as the plethora of rights typically conferred on citizens of a country.
You may apply if you find yourself in any of the following groups: where your parents/grandparents are Portuguese, if you have lived legally in the country for a period of 6 years or more (3 years or more if you marry a Portuguese citizen). Portugal permits dual citizenship, so you may retain your existing citisenship
Please note that there do exist some special rules and limitations so you should discuss the matter with an immigration lawyer to make sure you are eligible.