Séadnas
Note: I don't know what the plural for this is.
The séadna (shay-na) is an Irish 4-line, rhyming, syllabic form. There are key differences between the even- and odd-numbered lines: odd-numbered lines have 8 syllables, end with a 2-syllable word, and don't rhyme with each other; even-numbered lines have 7 syllables, end with a 1-syllable word, and do rhyme with each other (the rhyme pattern is A/B/C/B).
Furthermore: the last syllable of the first line should alliterate with the first stressed syllable of the second line; the third line should rhyme with the penultimate stressed syllable of the last line; and the main rhyme (between the even-numbered lines) should be echoed with internal rhymes or half-rhymes in the third and optionally fourth lines.
That's a lot, so here's a demo:
x–x–x–x–x–x–x–A/ d–d–d–d–d–d–B/ e–e–e–e–e–e–e–C/ f–f–f–f–f–f–B
Syllables marked “x” are unrhymed; the first stressed syllable marked “d” must alliterate with the syllable marked “A”; one of the syllables marked “e” and one marked “f” must rhyme with the “B” syllables' rhyme.
Finally, as is often the case for Irish forms, the first and final words of the poem should be the same word.