Why not some authentic historical incidents from his life? This was translated from an early Gaelic manuscript by Professor Dominic Behan.
I think.

(Tune - Rozzin the Beau)

When Caesar by conquest elated, on Erin's green slopes cast his eye,
He said to his troops, 'Be upseated' and went forth to the wake as a spy.
The hairy barbarian, the Briton, in England he'd conquered anew;
But he had some fresh fetters to fit on the freemen of Brian Boru.

Dressed in a pair of stout breeches, a good pair of boots and corbeen,
He struggled through hedges and ditches to where the wake lights could be seen.
But being so hale and so hearty he arrived rather soon in the night
Before the brave lads at the party were quite drunk enough for a fight.

King Brian Boru sat and eyed him, as also did huge Finn McCool,
While another was sat in a corner with his crosier propped up on a stool.
To Caesar a pipe was extended by him with the crosier and cloak;
But Caesar refused, most offended, saying, 'Cities must burn when I smoke.'

'Is that so?' said the other quite civil. 'You'll need a big pipe for that same.'
'Ah well then,' said Caesar, quite livid. 'I'll be damned if you know then my name.'
'Your name and your fame' said the other, 'would both be much safer at home;
For the bogs of old Ireland may smother a heathen come over from Rome.'

Caesar stood, wanting to leave then, but as soon as he made for the door,
He found to his very great wonder that his two feet were stuck to the floor.
'Who are you whose glances appal me?' The other replied with a smile:
'St Patrick my countrymen call me. I'm the guardian of Erin's green isle.'

Caesar stood seeming to linger, but Brian Boru bade him go.
St Patrick he lifted his finger and Finn McCool lifted his toe.
He shot from the spot like a rocket, for Finn McCool kicked with a will.
The troops on the shore felt the shock; it electrified valley and hill.

Away cross the billowy Biscay, so sickened and saddened he goes,
Convulsed with a craving for whiskey and a highly reduced Roman nose.