Will a publication in one subfield of my discipline help me get into graduate school in a different subfield? I am a psychology undergrad student. I am currently working on a project that will be submitted for publication later this year. However, the project I am working on is in the sub-field of psychology that is not of my interest (environmental psychology). I am planning to apply for PhD programs in social/personality psych. (Sadly I didn't get a chance to conduct a research project in that field.)

Although my publication could be a strong evidence for my research potential, my concern is that this publication is irrelevant to my research of interest. Thus, how can I convince the grad school that I am a good candidate for the social/personality field?
Even if your publication were in physics or economics or any other field, it would still be a positive component of your application. When I look at graduate applications, a publication record--while not essential--is valuable because it signals the following: 


Interest in doing research
Initiative to get into a lab and work on a project
Familiarity with what primary research is actually like
A close enough relationship to a faculty mentor for me to pay particular attention to that mentor's letter of recommendation


All of these hold in your case.