Well there are a couple of ways of doing it. One way is here which is a hack.   
How can I rewrite this SQL into CodeIgniter's Active Records?
This other way is very simple.
$this->db
        ->select('ID')
        ->from('table2')
        ->order_by('id')
        ->limit('5');   
$subquery = $this->db->_compile_select();
$this->db->_reset_select(); 
$query  =       $this->db
                    ->select('t1.name')
                    ->from('table1 t1 ')
                    ->join("($subquery)  t2","t2.id = t1.t2_id")
                    ->get('table1 t1');
Some point about it.
You are bound to use from clause in subqueries because get runs the query.
In codeigniter 2 _compile_select and _reset_select can not be accessed because they are protected methods.
You may have to remove the keyword before both methods in system/database/DB_active_rec.php 
This article is useful too.