The phrase “if able” means that you can choose an opponent with fewer than 2 cards in hand. That player will simply resolve as much of the effect as possible, whether that means putting their only card on top of their deck, or nothing at all. That player will still draw 2 cards at the end of the phase though, regardless of how many cards were placed on top of the deck.
If The High Sparrow is in play and the chosen opponent has already drawn 3 cards that round, they will not draw any cards at the end of the phase. If they have drawn 2 cards that round, they will only draw 1 for Fair Isle.
Placing cards on top of your deck is not the same as discarding those cards from your hand, so Fair Isle would not trigger a card like Cersei Lannister (LoCR).