Can an IAS officer go to MEA or an IPS officer join R&AW or IB — how does inter-service deputation actually work?

Yes — the Indian civil service permits structured deputation (not permanent transfer) of AIS officers to other ministries and intelligence agencies. IAS officers go to MEA primarily as Joint Secretaries or Additional Secretaries on tenure deputation, not as IFS replacements. IPS officers join the Intelligence Bureau (IB) on permanent absorption through the Central Staffing Scheme and R&AW through the Research and Analysis Service (RAS), which inducts officers from IPS, military and other services after a minimum of 5 years of service. Permanent inter-service transfers are not allowed; the AIS Rules permit only inter-cadre transfer within the same service (on marriage or extreme hardship).