HI Richard!
I recall you changed stems a short while ago. On a threadless headset, the stem (or the Thorn Accessory T-bar sometimes mounted below it) locks-int he headset adjustment.
It might be the headset is a bit loose, accounting for the lateral movement when shaking the front wheel sideways. This is what I would suggest checking first, as it seems most likely given the sequence of events.
Other things to check:
• Rack mounting bolts
• Front wheel q/r tightness (as already suggested). If it isn't fully seated beyond the "lawyer lips", it can appear tight but still be loose
• Pannier mounting hooks and tightness...*and* whether any sizing clips are missing (if your bags use the Ortlieb QR-2 mounting system)
• Front hub bearing adjustment (also suggested earlier)
All best wishes for an enjoyable Irish tour!
Dan.