You could progress onto a jig an hour after that and by the end of the day, you could be pretty competent, enough to raise the wife's eyebrow anyway!
But, an as the saying goes, Practice makes perfect! In order for you to become good, you have to keep playing, listen to the music so you have it in your head and then practice it slowly until you have it, then when you think you have it listen to other's playing, does it sound different? Of course!
Once you think you have it, you can learn about ornamentation, cuts, rolls and the way to blow, create your own style.
The tin whistle is one of the cheapest instruments you can buy and one of the most beautiful you may ever hear. Theres a very easy to follow tin whistle tutorial at http://www.whistleaway.com, you could be playing in days!