Google +1 is very easy to add to your site, you only need 2 things for a basic button.
1. Google’s JavaScript, make sure its is https not http!
`<script type="text/javascript" src="https://apis.google.com/js/plusone.js"></script>`
To add this to your site in your functions.php you can write,
function google_plus_gp() {
wp_register_script( 'google-plus', 'https://apis.google.com/js/plusone.js');
wp_enqueue_script('google-plus')
}
add_action ('wp_enqueue_scripts','google_plus_gp');
2. The markdown where you want the button.
<g:plusone></g:plusone>
You can add something like a content filter for after your posts.
add_filter('the_content', 'google_plus_plus');
function google_plus_plus($content) {
$content = $content.'<div class="plusone"><g:plusone href="'.get_permalink().'"></g:plusone></div>';
return $content;
}