Jirachi is a mythical Pokémon and is only obtainable through special events or distribution events. It was never available in the original SoulSilver game.
However, if you are playing Pokémon SoulSilver on a modern device like a Nintendo Switch, you might be able to obtain Jirachi through special events that Nintendo might organize for older games.
Keep an eye out for any announcements or updates about special events or distribution programs that might include Jirachi.