"Debut single release from ClariS featuring intro theme to anime series "Ore no Imōto ga Konna ni Kawaii Wake ga Nai" Limited edition includes a bonus DVD featuring music video of "irony." Includes a wide cap sticker type A. *The DVD disc is encoded for region 2 (Japan, Europe, and Middle East), and no subtitles are included." - CDJapan
"Debut single release from ClariS featuring intro theme to anime series "Ore no Imōto ga Konna ni Kawaii Wake ga Nai" This limited edition includes anime-size TV mix. Features anime illustration digipack." - CDJapan
Note that this is different to the previous "irony (First Press Limited Edition)". This for an example, doesn't contain a second disc with the "irony (Music Video)" and carries a different track called "IRONY -Instrumental-".
""Nexus" is a pop song by the Japanese duo and idol unit ClariS, written by Kz. It was released as the unit's third single on September 14, 2011, by SME Records. The song was used as the theme song for the ninth volume of the Oreimo light novels and the opening theme of the Ore no Imōto ga Konna ni Kawaii Wake ga Nai Portable ga Tsuzuku Wake ga Nai video game. A music video was produced for "Nexus", directed by Kazuaki Nakamura. The single peaked at No. 5 on Japan's weekly Oricon singles chart." - Wikipedia