As has occurred on numerous occasions during the past 100 years, moves to end abstentionism result in the more hardline republicans splitting. This is not in any way conducive to the peace process and coupled with the inevitable galvanising effect on the groups who still support military resistance, could be absolutely disastrous.
In the long term, the potential disintegration or division of their movement is vastly more significant to Sinn Fein - and arguably Ireland - than Brexit could ever be, regardless of the resultant short term disruption it will likely cause.