Lark in the morning is one of my all-time favorite jigs to play and to teach. It's a 4 part double jig in 6/8.
The length of this tune may appear daunting at first, but a lot of phrases simply repeat themselves, making it easier to play. In fact, the last two parts are more like variations than actual separate bits of the tune.
The long G and F which come at the end of all but the first part can be rolled to produce a nice effect. Alternatively, on the fiddle try double-stopping the G with a B and the D with an open A string.
This tune would seem to be a pipe tune so it probably lends itself to the whistle more than the fiddle.
This tune appears in The Cumann na bPiobairi Collection of Pipe-Friendly Tunes by John B. Walsh and Mel Bay’s Complete Irish Fiddle Player by Peter Cooper.
Peter Cooper relates a story associated with this tune about two fiddlers who had a contest to see who could be the best fiddle player. The fiddlers played all night until dawn broke and they could play no more and still it could not be decided who was the better player. In the silence of the morning, they heard a morning lark. Both fiddlers agreed that it wasn’t either of them that had the sweetest music, but "The Lark in the Morning".
Cooper suggests using turns on the consecutive G and F in the 2nd, 3rd and 4th part.