The US Copyright Office lists TRUE LOVE KNOWS NO SEASON with Norman Blake as copyright claimant, and BILLY GRAY (note spelling) as alternative title, 1986.

However, it appears as BILLY GREY (note spelling), with melody line in standard notation and tablature, plus chords, in The Norman Blake Anthology (Mel Bay Publications, 1998), from which I copied this: (Boldface indicates words that are different from Planxty's version.)

BILLY GREY
(Norman Blake)

Billy Grey rode into Gantry back in 'Eighty-Three.
There he did meet with young Sarah MacCray,
The wild rose of morning, the pale flower dawning,
The herald of springtime in his young life that day.

Sarah she could not see the daylight of reality.
In her young eyes Billy bore not a flaw,
Knowing not her chosen one, he was a hired gun,
Wanted back in Kansas City by the law.

Then one day a tall man came riding o'er the badlands
Lying to the north of New Mexico.
He was overheard to say he was looking for a Billy Grey,
A ruthless man and a dangerous outlaw.

And the deadly news came creeping to Billy fast sleeping
There in the Clarendon Bar and Hotel.
He fled toward the old church there on the outskirts,
Thinking that he'd climb up to the old steeple bell.

But a rifle ball came flying; face down he lay dying
There in the dust of the road where he fell.
Sarah she ran to him, cursing the lawman,
Excepting* no reason, knowing that he was killed.

Sarah lives in that old white frame house
Where she first met Billy some forty years ago,
And the wild rose of morning, she's faded with the dawning
Of every day of sorrow that the long years have sown.

And written on a stone where the dusty winds have long blown,
Eighteen words to a passing world say:
"True love knows no season, no rhyme nor no reason.
Justice is cold as the Granger County clay."

Yes, "True love knows no season, no rhyme nor no reason.
Justice is cold as the Granger County clay."

[* Excepting (sic) – should be accepting?]

* *
Allmusic.com says that when Norman & Nancy Blake recorded it on their album "Blind Dog" (1988), they called it BILLY GRAY (note spelling). Several other people recorded it under that title, too: Michael Martin Murphey, Robert Earl Keen, Mike Auldridge, Katy Moffatt, and a few others.

Planxty seems to be the only band or artist that recorded the song under the title TRUE LOVE KNOWS NO SEASON.

The lyrics appear in the DT under the title TRUE LOVE KNOWS NO SEASON, where it is apparently transcribed (with some errors) from the singing of Plaxty. See Bonnie Shaljean's post above for corrections.

They also appear in two other threads in this forum:

The thread Lyr Req: Billy Gray (Norman Blake) has a version transcribed from the singing of Robert Earl Keen, Jr.

The thread Lyr Add: Billy Gray / Billy Grey (Norman Blake) has a version from an unnamed source.

All these versions differ in some ways.