Gargoyle -
Reels Jigs and Hornpipes are all styles of dance. Each one has its original distinctive rhythm based on what the dancers feet should have been doing.

For a Jig, the rhythm is based on a basic step where the bar is divided into two groups of three notes. The main emphasis is on the first note in each group of three, so there are two "down beats" (I assume by down beat you mean as in a conductor's baton stroke, and not just a depressing tune). The pattern of the feet is one foot goes onto the ground on beat one (bearing the weight of the dancer) followed by the other foot making two taps on the ground (one forward one back) without the weight being put onto the tapping foot. Then for the second half of the bar, the weight changes to the second foot, and the first foot does the taps.(APOLOGIES TO ANY IRISH DANCERS for my description).
The six notes are all of equal length, but the emphasis is stronger on notes 1 and 4.

Most people these days do not actually play tunes for people to dance. They are either played for an audience to listen, or in a session where it is often just a background to the general conversation.
Your questions suggest you are wanting to play for listeners.
If you play for dancers, the purpose of the music should be to give a rock steady rhythm and "beat" which sets the pace of the dance and provides a "lift" whaich makes the dancing easier. To do this, the accompaniment does not need to be very interesting, it just needs the beats in the right places.
When playing for an audience, it is possible to vary the rhytm in a way which would disrupt real dancing, but still sounds nice.

A few people play music for listening, which still contains the "feel" of music played for dancing. Try to track down recordings of De Dannan or Altan, both are excellent Irish bands who play a lot of dance tunes which have kept the "feeling" but also stand up as tracks to listen to.
Quack!
Geoff the Duck.