WATCH: Irishman's song "O, America" performed at Trump's inauguration

Irish songwriter Brendan Graham was surprised to learn that his song "O, America" was performed at President Donald Trump's inauguration on Monday, January 20.

Graham wrote the song "O, America" for the Irish supergroup Celtic Woman, who recorded it in 2009.

On Monday, the song was performed by "America's tenor" Christopher Dean Macchio, who was accompanied by the Armed Forces chorus, before Trump took his oath of office.

Graham, 79, told RTÉ News on Monday that he didn't have the "slightest idea" that his song "O, America" was going to be performed at the inauguration.

He said he was out for a walk as Macccchio's performance of "O, America" began to be televised "and then the texts started flying in."

Graham said he was inspired to write "O, America" when he overheard the melody "Sweet Remembrance of You" while on his way to a showcase by composer William Joseph.

"I asked William would he allow me to put a lyric to it, which he agreed," Graham said.

Graham further told RTÉ News got his inspiration for the lyrics while journeying between Kinnegad and his home in Co Mayo.

"It seemed to suggest itself as a love song," Graham said, "but an anthemic love song to America.

"Most of it came fairly readily but to finish it was a bit of a wrestling match."

Graham said "thankfully" Joseph went along with his idea.

RTÉ News notes that Irish singer Anthony Kearns sang the song on Capitol Hill for former President Barack Obama and also for President Trump during his previous term.

You can watch Celtic Woman perform "O, America" here:

"O, America" lyrics

O, America, you're callin'
I can hear you calling me
You are calling me to be true to thee
True to thee, I will be

[Verse 1]
O, America, no weeping
Let me heal your wounded heart
I will keep you in my keeping
Till there be a new start

[Bridge]
And I will answer you, and I will take your hand
And lead you to the sun
And I will stand by you do all that I can do
And we will be as one

[Verse 2]
O, America, I hear you
From your prairies to the sea
From your mountains grand, and all through this land
You are beautiful to me

[Chorus]
And O, America, you're callin'
I can hear you calling me
You are calling me to be true to thee
True to thee, I will be

[Bridge]
And I will answer you (And I will answer you), and I will take your hand (And I will take your hand)
And lead you to the sun
And I will stand by you do all that I can do
And we will be as one

[Outro]
O, America, you're callin'
I will ever answer thee