Dear Krishna Venigopal,
Your query is too short. It does not give sufficient background information. Please confirm to us who will write the letter, to whom will it be written, why will it be written, who is the employee in question, what is the meaning of "being irregular" and so on.
Secondly, please write the letter on your own and upload the draft here. Some seniors will correct it. If you don't even write it, how will you improve your drafting skills?
Thanks,
Dinesh Divekar