Does EASA Part-IS apply to operators based outside the EU?

It can. Part-IS is an EU regulation, so it applies directly to organisations operating under the EASA system. If you are based outside the EU but hold an EASA approval, for example a maintenance organisation working under a bilateral agreement, you may still be expected to meet Part-IS or an equivalent standard.

The exact position depends on the terms of the bilateral agreement between your country and the EU, so it is worth confirming which EASA regulations are recognised for your specific approval. As a general rule, if your business touches the European aviation system through a certificate or approval, it is safer to assume Part-IS is relevant until you have confirmed otherwise.