Travelling from Ireland to Changchun in March.

What's your departure city? Anyway, you may fly to Beijing first, then take airport express train to Dongzhimen, change to subway line 2 to Beijing Railway Station. After, take a bullet train to your final destination. If there is flight to Shenyang, you can fly there, too; it closer to Changchun. You can search the train and flight schedule on this website on your own.

As for the weather, March and April is the early spring and it begins to turn warm with lots of sunny days. But you still need to wear thick clothes like thick jacket and sweater. Have a good time there~