Alanna is most familiar to modern English speakers as a female given name. It is derived1 from the Irish a leanbh, refering to someone as a child or baby2. Other common English given names, such as Alana and Elana have different origins, although most people do not attend to these factors when naming their children; Alana is currently the more popular name in America, presumably in part because of the straightforwardness in spelling.
A leanbh and Alanna are also used as a general term of endearment, both for children and romantic partners (akin to the English baby/babe/bae). It is now a bit archaic in English usage, with the heyday falling between 1830 and 1900. Modern usage is limited almost entirely to the given name. It is worth noting that leanbh works its way into a lot of Irish idioms, which can be confusing. You might see it as a term of endearment in leanbh mo chroí (child my heart), but the Anglization of this, lannamacree, is most often found in the phrase Lanna Macree's dog, refering to a highwayman.
A leanbh may be pronounced uh-LAN-uv or, in the Ulster accent, uh-LAN-nah. As far as I can find, no one pronounces Alanna with a final v.
1. Wikipedia, and therefor large swathes of the internet, claim without citation that Alanna may be derived "from the Old High German word for 'precious'". I have not been able to find any evidence for this.
2. Wikipedia, and therefor large swathes of the internet, also want us to believe that "in Gaelic Alanna is the term for 'beauty' or 'serenity'." This appears to be nonsense.