A big problem is that nobody seems to be monitoring Covid antibodies in a large-scale, consistent way. Here's the CDC's "COVID-19 Serology Surveillance Strategy" [1] They have a few tools. One is a blood donor test, where donated blood is tested for antibodies. Another is random sampling of blood tests taken for other medical reasons. Those establish bounds - blood donors are healthier than average, and people getting blood tests for medical reasons are less healthy than average.
The results, though, are very spotty. The most recent US data published seems to be from September 2020.[2] "In this repeated, cross-sectional study of 177 919 residual clinical specimens, the estimated percentage of persons in a jurisdiction with detectable SARS-CoV-2 antibodies ranged from fewer than 1% to 23%. Over 4 sampling periods in 42 of 49 jurisdictions with calculated estimates, fewer than 10% of people had detectable SARS-CoV-2 antibodies. ... Samples were obtained during 4 collection periods: July 27 to August 13, August 10 to August 27, August 24 to September 10, and September 7 to September 24, 2020."[3] Those are from anonymous random blood samples taken from people who had a lab test for something else.
For blood donors, there's this data.[3] "Data from the American Red Cross, which is providing serosurveillance data as part of the MASS study, indicated that 8.4 percent of U.S. blood donors it tested in a week in late November carried antibodies to Covid-19 in their blood."
So that's the US situation from November. Those set upper and lower bounds on how many people in the US have had Covid-19. Those are actuals, not estimates. One would expect this would be tracked and updated weekly, but if it is, the data isn't widely available. The main thing one can conclude from this is that it's nowhere near "herd immunity". That would require 7x-10x as many people showing antibodies.
The CDC's estimates based on a flu-based estimate of how many people need medical attention per infection seem inconsistent with that actual data.
As for the "Ireland event", here it is on the Financial Times graph with other countries.[4] From 5 cases per 100K to 130 cases per 100K. For comparison, California is now around 105 cases per 100K.
[1] https://www.cdc.gov/coronavirus/2019-ncov/covid-data/serolog...
[2] https://jamanetwork.com/journals/jamainternalmedicine/fullar...
[3] https://www.medscape.com/viewarticle/944035#vp_2
[4] https://ig.ft.com/coronavirus-chart/?areas=eur&areas=usa&are...