the rashbam brings the abbreviation meis"h which stands for rabbinu yashar gaon. I searched in the seder hadoros and other sefurim on that era and didn't find a mention of a gaon with this name.
Noah Zablotsky, New York, USA
The abbreviation stands for Rav Yisrael Gaon (as pointed out by the MAHARSHAL, see HAGAHOS MAIMONIYOS to Hilchos Nachalos, 9:8). According to Rav Refael Halpern (Atlas Etz Chaim, vol. 6 #226 -- I haven't found his sources) there wasa Rav Yisrael Gaon, son of Rav Shmuel bar Chofni Gaon, who was a brother in law of Rav Hai (last of the Geonim) and who authored a work on Hilchos Tefilah.