Lots of tunes work a lot better if you play on a whistle a fourth or fifth below the key, which is what Leadfingers is trying to say (he's over 60 now, so we must forgive him). So a D whistle is sometimes good for playing in G and A, and an A whistle is sometimes good for playing in D and E. If a tune is in a minor key, don't automatically reach for the relative major - try the whistle one tone below - so if the tune is in Em, a D whistle can work better than a G whistle.

I think what smallpiper is saying is that a lot of tunes have triplets which can be omitted - you can play the first note of the triplet as a quarter note and ignore the other two notes of the triplet, and the tune will still work. When your technique is better you can go back to inserting the triplets.

A lot of the faster whistle players stop the holes with the second joint of the finger, or the flat part of the top joint, rather than the fingertip (as is usually recommended in books). It is not as hard as it might seem to half-stop a hole this way, and can be a better way to play if it suits you personally.