Tight security as Charles and Camilla visit both sides of the Irish border

Spending some time in Northern Ireland is always an opportunity to remark on how it has transformed since the dark days of the 1970s and 1980s.

But that doesn't mean security for a royal visitor can be stood down.

Ahead of, and all around, the Prince of Wales and Duchess of Cornwall, you will find a sea of dark green police uniforms and lots of men and women in suits with earpieces.

In the sky, a spotter plane was circling above us.

But then again, this is the heir to the British throne and the Colonel in Chief of the Parachute Regiment - which fought with and killed protestors in Derry in 1972 in what became known as Bloody Sunday.

Charles and Camilla unveil a plaque at the memorial garden at the PNSI headquarters. Credit: PA

That said, the royal tour took Charles and Camilla to Derry/Londonderry yesterday - to the Protestant side of the River Foyle anyhow.

There they opened a new cancer hospital (which treats all members of the community as well as patients from over the border in the Irish Republic).

Any royal visit to Northern Ireland will be framed by the Troubles and its aftermath and that was inevitably the case when when the Prince and Duchess opened a memorial garden to fallen officers from the Police Service of Northern Ireland.

The PSNI was only formed in 2001, when it replaced the RUC, but 13 of its officers have been killed in the line of duty since then.

Two of the names on the memorial wall were those of Constable Stephen Carroll and Constable Ronan Kerr.

Constable Stephen Carroll's wife said he always 'had a target on his back'. Credit: ITV News

Both were murdered by dissident republicans: one shot by a sniper, the other blown up in his car.

And just this year - in January - there was an attempt at murder of a PSNI officer in Belfast. He was injured but survived.

It's a reminder, said the force's Chief Constable George Hamilton to us, that despite the peace process there are still serious risks for his officers when they are on duty on the streets of Northern Ireland.

The widow of Constable Carrol told us how she would worry everyday about her husband - who, she said, 'had a target on his back'.

The four-day tour here has now moved south - to the Republic of Ireland.

The Prince has met the President in Dublin and will later this week meet the Taoiseach here.

And the focus turns from past difficulties in the North of this island to the present difficulties in the South.

Charles and Camilla are welcomed to Dublin by President Michael D Higgins and his wife. Credit: PA

The Irish acknowledge they have more to lose from Brexit than any other EU country.

The biggest customer for Ireland's exports is - and will remain - the UK.

There is also the issue of the border between the Republic and Northern Ireland. We drove through it today and there is no border: it's a free-flowing motorway.

What no one can answer right now is how that 'Common Travel Area' is compatible with the UK leaving the EU and seeking to control all its borders more tightly.

Prince Charles may choose to address these issues here in his private meetings, but outwardly this tour to Ireland is about extending a hand of friendship.

As one Irish official just put it to me, "Well, you're not going anywhere are you?" and the Royals are here to remind our neighbours that there will still be a need for relationships even after Brexit has taken place