It's about 50-50 between Junction 4 (Swords, Donabate) and Junction 2 (Dublin Airport), as Airside is almost half-way between the two.
If you take Junction 4, you'll get onto the Swords bypass and all those roundabouts (which can be busy in morning rush hour).
If you take Junction 2, you'll have to negotiate the large Airport Roundabout which is always busy!
Junction 3 is an on-ramp only from Swords onto the M1, you can't exit for Swords travelling southbound.