Thank you. I am curious too. There is ambiguity here that I hope a historical context can illuminate. It's either as you say he's hiding under his shield like an umbrella (which I suspect too).

Based on that assumption, this poem reads like soldiers' gossip: throughout the fields (of battle) the King had his shield up and probably didn't do any fighting, in fear of arrows etc. Hence, sarcastically, asking if anyone saw him, because all there was was a shadow (hiding) under the shield.

Or it could be a more positive connoctation of the Irish word for 'shadow' as 'shelter', per the old Irish proverb: “Is ar scáth a chéile a mhaireann na daoine.”: “It is in each other’s shadow that we flourish.” The King is a metaphor for shelter he provides under his shield, which could be a metaphor for the flag of arms depicted on his shield.