Shannon change their sector frequencies quiet often during the day. They have about 6 or 7 which are regularily used for aircraft leaving the UK FIR at STU.
To the east of the boundary at STU the aircraft are on 129.375 below FL335 and 135.255 when the above. If the aircraft is routeing GATRA-EVRIN-BUNED then they will be coming from 132.950. Finally if UL18 is open, 133.6 is normally the frquency but sometimes it's also 135.255.
To the west of the boundary, Shannon use the following frquencies:
131.150
132.150
135.6
amongst others
Dublin are on 129.175 and 124.650. Don't forget that Dublin only controls below FL245, so you may get aircraft going to DUB but calling Shannon
Hope this helps