Hi Brian - Do you know for sure that your man was in the Manchesters? There is a P. O'Brien on the 1st Battalion roll and that's all. The 1st Battalion was, as you say, involved in the Defefnce of Ladysmith but the 2nd Battalion wasn't in the Natal theatre at all. However, about 130 detached men from the 1st Battalion were involved in the operations to relieve Ladysmith.
I've come across a number of family snippets of a man 'being in Ladysmith' but these could mean he was in the defence, in the relief force or in the Ladysmith garrison after the relief.
There were many units involved in the defence and relief of Ladysmith but as he doesn't appear on the medal roll of 1/Manchesters this strongly suggests he was in another regiment.
Regards, David