A Roinnard Tri-nard has the following characteristics:
It is a quatrain (four lines).
Hexasyllabic (six beats per line) ending in disyllables (a two-syllable word).
Alliteration in each line. (Same letter sounds, such as Luke likes to lick lakes, etc.)
The end of line 1 alliterates with the 1st accented word in line 2
It ends on same syllable/sound as it began.
The 2nd couplet (lines 3 & 4) has 2 cross-rhymes, but there are none in 1st couplet (lines 1 & 2).
Line 2 rhymes with line 4
Line 3 consonates with both 2 & 4 (a 'near rhyme' that isn't one quite).
The pattern would therefore look a bit like this:
x x x x xa*
x x* x x bc
x b x x xc#
x x c x bc
in which c# consonates and a* alliterates with x*, and the bold x & bc have the same sound.
Got that?
It's a bit of a stinker in English as it wasn't meant for the English language, but that's what makes it fun. Here are three I wrangled out during the day today - I had to cheat a little on the alliteration in the second one by hyphenating, and both could be improved, but how I haven't figured out yet.
Teatime
Feeling somewhat sluggish,
Mugs sip, mumble softly,
Tumblers fit for quaffing
Bitter half-black coffee.
Ghost-stories
Always told with terror,
To tales spoken, stalking
Fear, all shiver - fallen
Ghouls given life, walking.
Tolkien
Words upon white paper,
Punc-tuating towards
Old world order; shepherds
Faery swept to forward.
~ ~ ~
Your turn.
I totally can't write a poem of any sort and am definitely not going to try this one. You're the brave one Primula! 
BTW, how long did it take you to come up with that? That was great!
I think I got all the inards in the right places but not sure if the camel will live.
These little things really do make you have to roll up your sleeves and crack your knuckles a bit, don't they? I tend to feel as if I am trying to fold word-origami. 
