I'm going to answer what I can without dancing around an NDA, here, so bear with me. If I'm being too vague, I apologize. I'm trying to clarify as much as I can (to keep expectations in-line with what the product offers), but we also just received a stern lecture about NDAs, and I really don't want to get fired.
1) Yes, "Tir Tairngire" translates to "Land of Promise."
2) It is in some ways a sequel to the old Tir Tairngire book and the Tir's chapter in Sixth World Almanac, but in other ways it is meant to fill in the gaps between those two books. We've got several years of Tir history that were never touched on in any published material, and this is an attempt to fill in those plot holes, explain what's been going on, and basically update the setting and get it playable again. Having either/both of those would be helpful to really tackling the place.
3) Linked with the idea of playing there, this product and the Elven Blood adventures go together pretty well, and feed off each other a lot. While the print run of Elven Blood was a GenCon exclusive, it should be available on pdf eventually. So if you like Land of Promise and want adventures to go with it, Elven Blood gives you several of them.
4) I don't want to deflate anyone's hopes or shatter dreams, but please keep in mind Land of Promise is just a small, largely unsolicited (IE, it was the writer's idea to make the pitch, it wasn't the developer's idea to go looking for a book to be written), e-book only release. So there's limits on what can be crammed into this sort of book, the budget wasn't the budget available to a larger product, etc, etc. It's a pretty compact piece of work that serves primarily to update and supplement existing material, and just drag the Tir kicking and screaming into the 2070s and make it someplace fun to play in.