You could make a case that Old Irish is a 'Classical' language, like Latin or Greek or Sanskrit. Latin? After a while, ordinary Latin ends up being more or less transparent to the reader. (Ordinary Latin, not Tacitus or Propertius.) You can read it quite cheerfully. And Greek I think always remains a bit trickier (the midgy drifts of particles, the propensity to dialectical forms, the specialised jargons). And Sanskrit is like an exotic holiday for Classicists: a new script, a complicated phonology, the system of sandhi-variations which obscure the endings, and a general rather bewildering mixture of stylization and lushness. Like the above trio, Old Irish is Indo-European, has a heroic literature, and grammatical features such as inflected nouns and adjectives, plus a complex conjugated verbal system.
But describing the Old Irish verbal system as 'complex' is like referring to the Arctic as 'somewhat chilly'.
Yes. Indeed. I still can't wrap my head around Old Irish. Put Old Irish in front of me and ask me to translate it, and I might weep. Or send you on your way to David Stifter.
Oddly enough, the great Whitley Stokes could handle Old Irish, but apparently never mastered Modern Irish.
For those of you who love language, or Old Irish, go read it.