What you desire is doable, but somewhat complicated. There are two mesonet map code sets. The Global map uses data from the rollup of stations from all the regional networks (global-map.php) and so displays all stations (but without rotating conditions .. tried that, had browser(s) lock up from having too many DOM entities to manage).
The mesonet-map.php scripts address ONE network at a time (only) as they fetch data from the regional hub site for that network -- other networks are not included in the display which does include rotating conditions.
The European network (netID=ZEUR) is the only one that includes multiple countries in its data, so it is possible to see all the UK/Ireland members who have signed up for the European network. That may include stations from the UK network (or not). At present, there is no Ireland network (there was one associated with the regional networks, but we discontinued collection as it was taking 100sec to get the JSON data from the net, and the admin(s) were unresponsive to fix the issue despite repeated requests).
So, yes, you can modify the mesonet-map-inc.php to filter the data from the European network, and change the tab to display 'Ireland/UK' if you like.. the programming should be reasonably straight-forward. The data all comes from the JSON which includes country names for each station.
A simple approach to limit the default display to just Ireland/UK would be to set an override map lat/long/zoom to just show the countries with surrounding oceans.. then when the map opens, the main countries of interest would display.
Hope this helps...
Best regards,
Ken