Left side is
水府住玉川義久(花押)Tamagawa Yoshihisa of the Mito-han
Right side is
草木柏氏囗囗九種為 Kusagi Kashiwa shi ~ ~ kyushū no tame
The right side would be a dedication, presumably to the person who ordered the tsuba, but I'm not sure of the reading of the name. Kusagi (or Kusaki) and then Kashiwa, followed by 氏 which typically is used as a neutral honorific. Kusagi and Kashiwa are both viable last names, so their combination here stumps me. The following two kanji are a mystery. The final kanji, 為, is used to indicate a dedication (it basically means "for").