Along the west coast mountain biking spots are a bit thin on the ground, but there are a few scattered about. You could maybe fire an email to Moycullen Bike Works, as they are one of the bigger mtb shops on the west coast.
Scenery wise, I’d try and take in the Atlantic Drive in Donegal, making my way on to Slieve League (which you might go for a naughty spin around early in the morning). Then I’d make my way down into Sligo, and take a look around Yeates country, Benbulben etc (some secret trails here I believe) then I’d make my way on down to Achill via Killala.
You might be able to head back east via Ballyhoura, or Killarney depending on time. Or you could head home and check out Wicklow which is coming down with trails.