The "Irohanihoheto" in the title is actually the first line of a famous poem called the "Iroha" for short. The special thing about this poem is that each letter of the Japanese syllabary (kana) at that period was used only once in the entire poem, making it a sort of alphabet. This would make "Irohanihoheto" the equivalent of "ABCDEFG". The poem was in fact used as an alphabetical order for a time, and while this use has ceased now, it still remains in things like the numbering of seating for opera/theater houses, which is apt considering that many of the main characters in this show are part of a theatrical troupe. The line "Irohanihoheto" would be read in modern Japanese as "Iro wa nioedo", roughly meaning "the brilliant flowers", which is reflected in the opening animation.
Ichimura Tetsunousuke was a real historical figure of the Shinsengumi, although he is often thought to be a made up character. He is also featured in Peacemaker Kurogane as the protagonist.