<p:commandButton id="btnAddEntry" value="Add new row" actionListener="#{myBean.addNewCar}" ... update="carTable growl" process="@this carTable ..."/>

<p:dataTable id="carTable" var="car" value="#{myBean.cars}" ... editable="true">
        <p:column ...>
            <p:cellEditor>
                ...
            </p:cellEditor>
        </p:column>
    ...
        <p:column ...>
                <p:rowEditor />
        </p:column>
    ...
</p:dataTable>

public void addNewCar() {

    Car newCar = new Car();
    cars.add(newCar);

    FacesContext facesContext = FacesContext.getCurrentInstance();
    UIComponent uiTable = ComponentUtils.findComponent(facesContext.getViewRoot(), "carTable");
    DataTable table = (DataTable) uiTable;

    final AjaxBehavior behavior = new AjaxBehavior();
    RowEditEvent rowEditEvent = new RowEditEvent(uiTable, behavior, table.getRowData());
    rowEditEvent.setPhaseId(PhaseId.UPDATE_MODEL_VALUES);
    table.broadcast(rowEditEvent);
}

oncomplete="jQuery('.ui-datatable-data tr').last().find('span.ui-icon-pencil').each(function(){jQuery(this).click()});

RequestContext.getCurrentInstance().execute("jQuery('span.ui-icon-pencil').eq(" + rowToEditIndex + ").each(function(){jQuery(this).click()});");

styleClass="myTable"

oncomplete="$('.myTable tbody.ui-datatable-data tr:last-child td span.ui-row-editor span.ui-icon-pencil').click()"

<p:commandButton id="btnAddEntry" value="Add new row" actionListener="#{myBean.addNewCar}" ... update="carTable growl" process="@this carTable ..." oncomplete="$('.myTable tbody.ui-datatable-data tr:last-child td span.ui-row-editor span.ui-icon-pencil').click()"/>

<p:dataTable styleClass="myTable" id="carTable" var="car" value="#{myBean.cars}" ... editable="true">
        <p:column ...>
            <p:cellEditor>
                ...
            </p:cellEditor>
        </p:column>
    ...
        <p:column ...>
                <p:rowEditor />
        </p:column>
    ...
</p:dataTable>

oncomplete="jQuery('.ui-datatable-data tr').last().find('span.ui-icon-pencil').each(function(){jQuery(this).click()});"

oncomplete="jQuery('#tableForm\:table .ui-datatable-data > tr').last().find('span.ui-icon-pencil').click();"

jQuery('#FormID\:DataTableID\:#{datatable.size()}\:rowEditorID').find('span.ui-icon-pencil').each(function(){jQuery(this).click()});

Oncomplete="jQuery('#FrmID:TabViewI:mydataTableID'.replace(/:/g,'\:')).find('span.ui-icon-pencil').each(function(){jQuery(this).click()});"

oncomplete="jQuery('#FrmID\:TabViewID\:mydataTableID').find('span.ui-icon-pencil').each(function(){jQuery(this).click()});"