"She returned and she left again."
Translation:D'fhill sí agus d'imigh sí arís.
Because it was originally two words (a rís, to be precise; compare Scottish Gaelic a-rithist), like anseo (ann seo) and aniar ("an iar"). Note that Irish words are always stressed on the first syllable, too - except for compounds like arís. See here: http://www.irishgaelictranslator.com/translation/topic104727.html And here: http://www.daltai.com/discus/messages/13510/55167.html?1299023262
Just in case, ae is always classed as broad - to have a slender vowel after, it needs to be aei. e never occurs immediately before a consonant in any other circumstance - it's always rendered ea or ei.
You mean when Munster speakers stress syllables with long vowels even if not initial? I supposed that was a glaring omission seeing as I'm particularly going after Munster - GRMA!
While I'm at it, I should probably say that prepositional pronouns like agam and chugaibh are stressed on the second syllable too.
Not really. It's fairly common in spoken Irish, but in written Irish it is largely relegated to set phrases (such as numbers (bliain is fiche - "21 (years old)", a dó is a trí, sin a cúig), and of course, it's not unusual in songs, where it's single syllable might be a better fit for meter than the 2 syllables of agus. But in most written Irish, it's usually written out as agus.