HTML:

<ion-item>
    <ion-label stacked>Label do campo</ion-label>
    <ion-input type="text" formControlName="nomeCampoNoForm"></ion-input>
</ion-item>
<ion-item *ngIf="!form.controls.nomeCampoNoForm.valid && (form.controls.nomeCampoNoForm.dirty || form.controls.nomeCampoNoForm.touched)" color="danger">
    <div [hidden]="!form.controls.nomeCampoNoForm.errors.required">
        O campo é obrigatório!
    </div>
</ion-item>

TS:

import { FormBuilder, FormGroup, Validators } from '@angular/forms';

export class ExemploPage {
    item: any;
    form: FormGroup;
    constructor(private formBuilder: FormBuilder) {
        this.item = this.navParams.data.item || {};

        this.form = this.formBuilder.group({
            nomeCampoNoForm: [this.item.nomeCampoNoForm, Validators.required],
        })
    }
}