The Assassin's creed series has several games on PsP,PsP Vita and Nintendo DS. Does the lore about Desmond continue in these games? Do you get a new insight about him and his ancestors? Or are the games completely irrelevant to the series on Xbox 360/One(AC1,AC2,AC Brotherhood,AC Revelations, AC 3, AC4,AC Unity)
The game on PSP was Assassin's Creed: Bloodlines and it follows Altair after the events of AC 1. It does not continue Desmonds Story because, well, AC 2 does that. It starts off Desmonds story right after AC 1.
PSVita was Assassin's Creed: Liberation which again, as nothing to do with Desmond and the Nintendo DS ones I believe they don't either.
I wouldn't say they are irrelevant. AC: Bloodlines explains a lot of stuff from AC 2. Especially the creation of the Codex. AC: Liberation has an interesting plot with Aveline and the connection with Eve (and she is featured in AC 3 in a DLC Mission with Connor and as a playable character of a AC 4 Black Flag DLC). The Nintendo DS ones are the most irrelevant ones. They don't add much to the overall Lore of the Desmond Saga.
They might not be directly connected to Desmond or not reference him at all but they are really interesting and fun games that every fan of the series should try.