A weir appears prominently in the story of Taliesin, the great Welsh bard. Now there are two versions of the story. (by the way, I am speaking from memory and dont speak Welsh so a few names may be a bit wrong).
In one version Ceridwen is chasing little Gwion Bach, from llanfair Cereinion(a rather prosaic little place now, but perhaps more romantic in Ye Olde Days). anyway, after a lot of Celtic stuff, drops of poison, chewing his thumb for wisdom, changing into things and stuff, he eventually turns into a grain of corn. Quick as a flash ceriwen turns into a hen and eats gwion. This makes her pregnant, and she gives birth to to Taliesin(quite whym when it was gwion she swallowed, I'm not sure). Anyway, bear with me, we are getting to the weir bit. She puts the baby Taliesin into a leather bag and chucks him in the river, and he ends up on May morning hanging on a pole at the Garod Gwyddno Garanhir: that is to say, the fish weir of King Gwyddno Lonlegs. This weir is on the Dovey estuary, between Aberdovey and Aberystwyth.
    Now others tell the story quite differently, and have Taliesin going to visit Urien, King of Rheged(which is Cumbria in NW England now), and getting captured by Irish pirayes (same thing that happened to St Patrick). Anyway, out in the Irish Sea he sees an empty coracle floating by. he jumps out of the pirate boat and escapes in the coracle, and ends up(you've guessed it) hanging high and dry in the coracle as the tide goes out, on a pole at the fish weir of Gwyddno etc etc.
    The only other thing I know about weir is that Weir(English), Garod(Welsh, hope Ive spelt it right, might be Garog or Garoch or something), Crewe (town in Cheshire) and Penkridge(the kridge bit, town in Staffordshire) all share the same etymology, all meaning fish trap.( W,C and C transmuting freely into each other in the various Indo-European languages, just like Welsh, Gael and Celt).