In current IRIS versions, it's named intersystems-jdbc-3.0.0.jar

In current Caché versions it's named cache-jdbc-2.0.0.jar

In both, you'll find it under: [INSTALL DIR]/dev/java/lib/JDK18

It's also worth noting that under IRIS the JDBC URL and the driver classname have changed. See this doc for details.

com.intersystems.jdbc.IRISDriver
jdbc:IRIS://127.0.0.1:51773/USER