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She'll probably be happy once she gets married.
Literal
She [topic-は] [if/when-marries-したら] happiness become [probably-でしょう].
The conditional ~たら frames marriage as a future condition: 'when/if she gets married.' Among Japanese conditionals, ~たら is the most flexible — it handles specific one-time conditions, sequence ('when'), and counterfactuals. 幸福になる ('become happy / find happiness') uses the noun 幸福 (more formal than 幸せ) plus になる for change of state. でしょう hedges the prediction as the speaker's guess rather than asserted fact, the polite cousin of だろう.