According to Official Guinness Records,
The fastest marathon dressed as a leprechaun (male) is 2 hr 59 min 30 sec and was achieved by Adam Jones (UK) at the Virgin Money London Marathon in London, UK, on 26 April 2015.Jones was one of 34 records verified by GWR on the day of the Virgin Money London Marathon.
For a complete list of 2015 records, please visit 2015 Guinness Records in United Kingdom.