As you said you have clear IES exam and want to join DRDO as engineer but as per me DRDO recruits through its own examination process and is not linked to UPSC, so if you want to join it then you have to appear in its recruitment exam

Here I am providing Defence Research and Development Organisation (DRDO) engineer recruitment info :

Vacancy Details:
Total No. of Posts: 163
Name of the Posts: Scientist ‘B’ & Scientist/ Engineer ‘B’
I. Screening Based on Valid GATE 2015/ 2016 Score: 154 Posts
Name of the Subject/ Discipline:
1. Electronics & Comm. Engg: 40 Posts
2. Computer Science & Engg: 35 Posts
3. Mechanical Engg: 26 Posts
4. Mathematics: 07 Posts
5. Electrical Engg: 10 Posts
6. Physics: 06 Posts
7. Aeronautical Engg: 05 Posts
8. Chemical Engg: 09 Posts
9. Chemistry: 05 Posts
10. Textile Engg: 02 Posts
11. Civil Engg: 08 Posts
12. Material Science & Engg/ Metallurgical Engg: 01 Post
II. Screening Based on Percentage of Marks in Essential Qualification: 09 Posts
Name of the Subject/ Discipline:
1. Agriculture Science: 01 Post
2. Animal Science: 01 Post
3. Cognitive Science: 01 Post
4. Biomedical Engg: 01 Post
5. Fire Tech Safety Engg: 05 Posts

Key dates
Start Date for Submission of Online Applications 20-03-2016 at 17:00 hrs
Last Date for Submission of Online Applications 10-04-2016 at 17:00 hrs
Last Date for Submission of Online Applications for Candidates Qualifying Degree in Final Examination 10-04-2016
Tentative Date of Interview for Subjects/ Disciplines in Part I May 2016
Tentative Date of Interview for Subjects/ Disciplines in Part II August 2016

DRDO Scientist/ Engineer recruitment