It’s true, tens of thousands of people arrived in RoI from the U.K. over Christmas. In my town about 1 in every 10/15 cars over Christmas had an NI reg.
300 border crossings versus what 10-15 ports & airports carrying passengers onto the island? Much more efficient approach is a co-ordinated all island approach. Hopefully as restrictions are eased any outbreaks can be contained with local lockdowns after that. The current Garda approach is snap short term checkpoints around the country (no more long tailbacks on the major routes with larger scale checkpoints as well as the snap ones).
also sealing the border in 2001 required thousands of Gardai and troops, that are required around the rest of the country (there was still an outbreak on 1 farm in RoI due to smuggling across the border)
the good news is Irish restrictions have stabilised the increase. In the last 2 weeks, there was 4 days where there was over 6,000 new cases and 1 day where there was over 8,000. The last 4 days have had under 4,000 each day.