The Coordinate System is Irish Transverse Mercator:
https://en.wikipedia.org/wiki/Irish_Transverse_Mercator
In the downgraded system at
https://webapps.geohive.ie/mapviewer/index.html
one must click the icon near the top right for "Zoom To Coordinate" and manually enter (or at least copy-and-paste) the separate Easting and Northing coordinates, for example 504135 and 631212 respectively from
http://maps.osi.ie/publicviewer/#V1,504135,631212,6,7
Then one can click the icon near the top left for Basemap Gallery and select, for example, MapGenie 6 Inch First Edition Colour.
However, none of these clicks is reflected in the URL, so it is unclear how one can bookmark the exact map location as one could before the better system evaporated!
While there are only 10 of these broken links on WikiTree, there are hundreds or thousands of others on other websites (and especially in my offline genealogy database in profiles which still have to be uploaded to WikiTree!).
There is a converter at
https://www.fieldenmaps.info/cconv/cconv_ie.html
which tells me that the point of interest in this example, using the WGS84/ETRS89 coordinates, is at
https://www.openstreetmap.org/#map=13/52.42399165/-9.40950393
and
https://www.google.com/maps/@52.42399165,-9.40950393,13z
but neither of those links puts a marker at the exact point on the map as the original maps.osi.ie did.
I can add a marker at the exact point in Google Maps, but I can't see any way to bookmark or share that marker - the Share link seems to bring one to New Orleans (the first pin that I added to my map) instead of to County Kerry (the most recent pin that I added to my map):
https://www.google.com/maps/d/u/0/edit?mid=13rpECXWc6cGBVMz6UaA9SBJc83A&usp=sharing
I was hoping that there might be a way to create a link to a marker at an exact point of interest at
https://plus.wikitree.com/maps