You will have to have a garda witness your signature on a replacement form - they usually have the forms too so pop in there- the fee is about €6 (maybe its gone up....)
this form is then either presented to your local motor taxation office or sent to the dept. of the environment in shannon , co clare.
If you require a replacement document for any of the following, Form RF134 must be completed and witnessed by a member of the Garda Siochana at a Garda station and presented or posted with the appropriate fee to your Motor Tax Office.