Not sure if LDS site is acting up again but no matter what I try I cannot get this entry to come up in the results, so I searched on the original
www.familysearch.org site and found the entry.
Perhaps someone else will be able to find it in the civil registration index to get you the volume number to order the certificate.
Added- checking the indexes from another source the only 1864 births for a William Long (with/without middle names) are in Dingle, Croom,. Mallow, Newtownards, Balrothery, Athlone, Lurgan, Youghal, Dublin North.
I'm wondering if the record you found in LDS Irish births has the incorrect year as it's strange 2 sources for the index do not seem to show it.