Due to the underlying religious ties and affiliations of the clubs, Glasgow Celtic and Rangers are known to have considerable support within Northern Ireland. Celtic's Irish Roman Catholic foundations are well regarded and as a result, a large contingent of fans travel the short distance from Northern Ireland to Scotland for Celtic fixtures.
The same journey is made by Northern Ireland based Rangers fans. These two groups have had notorious altercations in the past.
The trip from Belfast or Larne to Cairnryan can be made by ferry. According to one recent Reddit post, these ferries had a novel tactic to deploy as crowd control:
We used to get sick passengers fairly regularly. Even in calm seas. When I used to work on the Irish Sea ferries, we would take a lot of the football fans heading to and from the Rangers/Celtic matches in Glasgow. If they started to get unruly, the bridge would call the engine room and ask us to retract the stabilisers.
Using the CCTV, you could see in a matter of minutes, people finding somewhere quiet to sit as the increased motion combined with their enthusiastic alcohol consumption soon lead to the rapid onset of seasickness. The lead steward always said he would rather clean up puke than blood.
The poster went on to clarify that the vast majority of the time fans were well behaved and this intervention wasn't usually necessary. Fans did clash on a ferry in 2005 which saw crew members sustain minor injuries. Martin Fletcher reports in his book 'Silver Linings: Travels Around Northern Ireland' that ferry companies will now only take one set of fans or the other, but never a mix.
However as this Reddit poster explained, there is always an unconventional plan B.