Design study for a narrow bandwidth, high power IRFEL has been carried out at NSRRC. This device will be synchronized with the Taiwan Light Source (TLS) storage ring for two-colour experiments. Instead of using lower gradient super-conducting linacs at UHF frequency, it has been proposed to use a high gradient 1300 MHz TESLA super-conducting linac to accelerate a high average current electron beam up to 60 MeV to drive this FEL oscillator. To obtain narrow bandwidth, the beam is stretched in bunch length by using a chicane decompressor before it passes through the U5 undulator of the FEL. Operating this linac in energy recovery mode is beneficial to improve system stability, wall-plug efficiency and to reduce neutron generation at beam dump. This device will be equipped with a low emittance injector for future upgrade towards shorter wavelengths. The ideas of the TESLA linac plus bunch stretcher scheme as well as a new type of low emittance high average current thermionic DC gun will be introduced in this report.