Hi buddy - gave it a go

1-redditexample.go

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"time"
)

// Fulldata - test
type Fulldata struct {
	Fulldata []Dataset `json:"data"`
}

// Dataset - test
type Dataset struct {
	IP      string    `json:"ip"`
	ID      string    `json:"id"`
	Time    time.Time `json:"time"`
	Smethod string    `json:"smethod"`
	Level   string    `json:"level"`
	Data    string    `json:"data"`
}

var testdata = `{
    "data": [{
        "ip": "192.168.2.1",
        "id": "1",
        "time": "0001-01-01T00:00:00Z",
        "smethod": "This is Method",
        "level": "Level 1",
        "data": "This is data"
    }, {
        "ip": "192.168.2.1",
        "id": "2",
        "time": "0001-01-01T00:00:00Z",
        "smethod": "This is Method2",
        "level": "Level 2",
        "data": "This is data2"
    }, {
        "ip": "192.168.2.2",
        "id": "3",
        "time": "0001-01-01T00:00:00Z",
        "smethod": "This is Method2",
        "level": "Level 3",
        "data": "This is data3"
    }]
}`

func main() {

	// Instantiate struct as variable
	mydata := Fulldata{}

	// Data converted to byte for unmarshalling
	bytedata := []byte(testdata)

	// Unmarshal data
	json.Unmarshal(bytedata, &mydata)

	// Do the magic
	filteredslice := filterjson(mydata.Fulldata, "192.168.2.1")

	// Complete struct and marshal back into JSON
	mydata.Fulldata = filteredslice

	K, err := json.MarshalIndent(mydata, "", "  ") // Pretty printing (using MarshalIndent with 2 spaces, rather than just Marshal)
	if err != nil {
		log.Fatal(err)
	}

	finaljson := string(K)
	fmt.Println(finaljson)

}

func filterjson(datasetslice []Dataset, ipvalue string) (filteredset []Dataset) {

	// Loop through datasets for values
	for _, element := range datasetslice {

		// If the IP matches, append the dataset to a slice
		if element.IP == ipvalue {
			filteredset = append(filteredset, element)
		}

	}

	return filteredset
}


// If you wish to see something a little  more complex, see this: https://gist.github.com/arehmandev/87a63bf5964ce4a8833294fb831c6373

2-seperate.go

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"time"
)

// Fulldata - test
type Fulldata struct {
	Fulldata []Dataset `json:"data"`
}

// Dataset - test
type Dataset struct {
	IP      string    `json:"ip"`
	ID      string    `json:"id"`
	Time    time.Time `json:"time"`
	Smethod string    `json:"smethod"`
	Level   string    `json:"level"`
	Data    string    `json:"data"`
}

var testdata1 = `{
        "ip": "192.168.2.1",
        "id": "1",
        "time": "0001-01-01T00:00:00Z",
        "smethod": "This is Method",
        "level": "Level 1",
        "data": "This is data"
	}`

var testdata2 = `{
        "ip": "192.168.2.1",
        "id": "2",
        "time": "0001-01-01T00:00:00Z",
        "smethod": "This is Method2",
        "level": "Level 2",
        "data": "This is data2"
	}`

var testdata3 = `{
        "ip": "192.168.2.2",
        "id": "3",
        "time": "0001-01-01T00:00:00Z",
        "smethod": "This is Method2",
        "level": "Level 3",
        "data": "This is data3"
    }`

func main() {

	// Instantiate struct as variable
	mydata := Fulldata{}
	mydataset1 := Dataset{}
	mydataset2 := Dataset{}
	mydataset3 := Dataset{}

	// Data converted to byte for unmarshalling
	bytedata1 := []byte(testdata1)
	bytedata2 := []byte(testdata2)
	bytedata3 := []byte(testdata3)

	// Unmarshal data into a Dataset
	json.Unmarshal([]byte(bytedata1), &mydataset1)
	json.Unmarshal([]byte(bytedata2), &mydataset2)
	json.Unmarshal([]byte(bytedata3), &mydataset3)

	// Create slice []Dataset
	unfilteredslice := []Dataset{mydataset1, mydataset2, mydataset3}

	// Do the magic
	filteredslice := filterjson(unfilteredslice, "192.168.2.1")

	// Complete struct and marshal back into JSON
	mydata.Fulldata = filteredslice

	K, err := json.MarshalIndent(mydata, "", "  ") // Pretty printing (using MarshalIndent with 2 spaces, rather than just Marshal)
	if err != nil {
		log.Fatal(err)
	}

	finaljson := string(K)
	fmt.Println(finaljson)

}

func filterjson(datasetslice []Dataset, ipvalue string) (filteredset []Dataset) {

	// Loop through datasets for values
	for _, element := range datasetslice {

		// If the IP matches, append the dataset to a slice
		if element.IP == ipvalue {
			filteredset = append(filteredset, element)
		}

	}

	return filteredset
}