this gist is gathered from here or there in the internet, and I made a few adjustment to support all of Ion getInstance/getDefault models.

Ion Set Self Signed SSL

    public void setSelfSignedSSL(Context mContext, @Nullable String instanceName){
        try {
            CertificateFactory cf = CertificateFactory.getInstance("X.509");
            // cert file stored in \app\src\main\res\raw
            InputStream caInput = new BufferedInputStream(mContext.getAssets().open("certificate.cer"));

            Certificate ca = cf.generateCertificate(caInput);
            caInput.close();

            KeyStore keyStore = KeyStore.getInstance("BKS");
            keyStore.load(null, null);
            keyStore.setCertificateEntry("ca", ca);

            String tmfAlgorithm = TrustManagerFactory.getDefaultAlgorithm();
            TrustManagerFactory tmf = TrustManagerFactory.getInstance(tmfAlgorithm);
            tmf.init(keyStore);

            TrustManager[] wrappedTrustManagers = getWrappedTrustManagers(tmf.getTrustManagers());

            SSLContext sslContext = SSLContext.getInstance("TLS");
            sslContext.init(null, wrappedTrustManagers, null);

            AsyncSSLSocketMiddleware sslMiddleWare;
            if(TextUtils.isEmpty(instanceName)){
                sslMiddleWare = Ion.getDefault(mContext).getHttpClient().getSSLSocketMiddleware();
            }else {
                sslMiddleWare = Ion
                        .getInstance(mContext, instanceName)
                        .getHttpClient().getSSLSocketMiddleware();
            }
            sslMiddleWare.setTrustManagers(wrappedTrustManagers);
            sslMiddleWare.setHostnameVerifier(getHostnameVerifier());
            sslMiddleWare.setSSLContext(sslContext);
        }catch (Exception e){
            e.printStackTrace();
        }
    }

    private HostnameVerifier getHostnameVerifier() {
        return new HostnameVerifier() {
            @Override
            public boolean verify(String hostname, SSLSession session) {
                return true;
                // or the following:
                // HostnameVerifier hv = HttpsURLConnection.getDefaultHostnameVerifier();
                // return hv.verify("www.yourserver.com", session);
            }
        };
    }

    private TrustManager[] getWrappedTrustManagers(TrustManager[] trustManagers) {
        final X509TrustManager originalTrustManager = (X509TrustManager) trustManagers[0];
        return new TrustManager[]{
                new X509TrustManager() {
                    public X509Certificate[] getAcceptedIssuers() {
                        return originalTrustManager.getAcceptedIssuers();
                    }

                    public void checkClientTrusted(X509Certificate[] certs, String authType) {
                        try {
                            if (certs != null && certs.length > 0){
                                certs[0].checkValidity();
                            } else {
                                originalTrustManager.checkClientTrusted(certs, authType);
                            }
                        } catch (CertificateException e) {
                            Log.w("checkClientTrusted", e.toString());
                        }
                    }

                    public void checkServerTrusted(X509Certificate[] certs, String authType) {
                        try {
                            if (certs != null && certs.length > 0){
                                certs[0].checkValidity();
                            } else {
                                originalTrustManager.checkServerTrusted(certs, authType);
                            }
                        } catch (CertificateException e) {
                            Log.w("checkServerTrusted", e.toString());
                        }
                    }
                }
        };
    }