A surprisingly simple way to skip over an entire third of the game. Just put an Escapipe in Wren or Mieu's inventory, and then as soon as Nial's story begins, use it. You'll end up back in Cille castle from Rhys' story. Go through again and fight King Cille again (should be relatively simple with your power level) and you trick the game into giving you Nial's marriage options.
This glitch is also _technically_ possible within Ayn's story, but you won't want to do it with him - doing so causes you to skip an item necessary to complete Sean and Crys' stories (the Twins' Ruby), putting the game in an unwinnable state.