Hi,
as someone who has monitored HF from South West Ireland, I can tell you that HF has fallen out of favor with many airlines who now use CPDLC (Controller to Pilot Data Link Control) i.e. satellite based ACARS-like position reports.
However, at present, most of them have to transmit position reports at 30degrees W, so after you hear them leaving Gander and checking SELCALs, you can calculate when they will be over the 30 degree mark. They then enter Shannon at 135.5, 132.15 or 131.15, which should be covered by Shannon ATC feed. The reverse happens over the other side. Shannon HF frequencies are 5616 and 5598 or 8864 and 8798. You can get the polar flights at 8871/4675 (all USB). The pole is not supported by CPDLC so HF is the norm up there.
Another way to monitor airlines over the atlantic is to use HFDL (download the free HFDL software from Charles Brain - search for Charles Brain HFDL) and use a USB radio selecting the frequencies for either Iceland or Shannon. It works very well and gives you postition information.
Peter