A few points regarding ornaments.
Whatever ornament you're trying to play, it's really all in the timing. In the context of Irish music, the ornaments we play on the fiddle are imitations of ornaments played on the pipes. Ornaments on the pipes evolved from the practical necessity of separating notes of the same pitch on an instrument which lends itself more naturally to legato than staccato playing; there's a constant supply of air moving through the chanter (the pipe where the melody is fingered) and it's impossible to start and stop that air movement crisply using the bag. There's no piper's equivalent of changing bow direction. A good piper can open and close the end of the chanter against his thigh, but the easiest way to separate two notes of the same pitch is with another note or notes.
I haven't included a time signature in all the examples below. The ornaments can be used in any kind of tune. If you're playing the same pitch for a duration of more than three 1/8 notes, you almost always break it up with one of these ornaments - if you choose not to, it will be for deliberate effect. If you have a note of the same pitch for a duration of three 1/8 notes, you will mostly use an ornament. A value of two 1/8 notes will often be separated, but by no means always. I'm deliberately avoiding talking in terms of 1/4 notes or dotted 1/4 notes, because the ornaments can be used across the barline, as well as within a measure.
Cut/Tip
These are the simplest ornaments:
The pitch of the grace note isn't necessarily important, because it's played so quickly it barely sounds, if it sounds at all. If it's above the note which it divides then it's a "cut"; if it's below, it's a "tip" or a "pat". I almost always play cuts with my 4th finger. I don't really use tips, but when I've tried them I've not played an actual grace note, but instead I've separated a note by relaxing pressure on the string momentarily, just to the point where the note stops sounding.
The whole thing is played in a single bow, and you can speed the bow up after the grace note for emphasis (eg when cutting a note across a barline). To get the timing of a cut, imagine you're playing left hand pizzicato, but of course you don't actually pluck the string.
Long Roll/Short Roll
A long roll takes up the space of three 1/8 notes:
A roll is usually described as "the note, the note above, the note, the note below, and the note again" (the hop, the cut, the hop, the tip and the hop). The pitch of the upper and lower notes isn't too important if you're playing the roll fast. The second measure above shows a typical open string roll.
The use of 1/32 notes isn't to be taken too literally. The important thing is that you're using the roll to separate the last 1/8 note. You can squeeze the fast notes in as quick as you like, as long as you squeeze them up towards the last 1/8 note and away from the first.
A short roll is the same as a long roll, but takes up the space of two 1/8 notes. So it's the same as the examples above but without the first 1/8 note each time.
Triplet
A triplet in Irish music is not the same as a triplet in classical music, which is perhaps why it's sometimes called a treble. The three notes of a triplet don't have to be evenly spaced. Here's the first measure of Drowsy Maggie twice, second time illustrating a triplet:
