JS:

$('.btn_measurements_login').click(function() {

	var email = $('.login_email').val();
	var password = $('.login_password').val();

	// Try to log the user in...
	$.ajax({
		type: "POST",
		url: "/Users/ajaxLogin",
		data: {email: email, password: password},
		success: function(html){
			if(html == 'true')    {
				alert('logged in...');
			} else {
				alert('fail...');
			}
		},
		beforeSend:function() {
			$('.login_error').html("<img src='/img/loading.gif'  class='loading'>").show();
		}
	});
});

PHP:

public function ajaxLogin() {
	$this->layout = 'ajax';

	$tmp['User']['email'] = $this->request->data['email'];
	$tmp['User']['password'] = $this->request->data['password'];

	$this->log(print_r($tmp, TRUE), 'l');

	if ($this->Auth->login($tmp)) {
		$this->log("UID: " . $this->Auth->user('id'), 'l');
		$this->log("Logged in...", 'l');
		echo 'true';
	} else {
		$this->log("Failed...", 'l');
		echo 'false';
	}
}