Unless I have a completely different version of My Heart's Tonight in TEXAS, they are totally unrelated, nothing alike whatsoever. Here's the version I have for the Texas song:

X:1
T:My Heart's Tonight in Texas
C:
L:1/8
Q:240
M:4/4
K:C
z6 G3/2A/ | BB BA Bd dB | AG GE G2 G3/2A/ | BB BA Bd2d | dB GB A2 G3/2A/ | BB BA Bd dB | \
AG GE G2 G3/2G/ | AA AA B3A | GG/G/ EE G4 | z8 | z8 | z8 | z8 | z8 | z8 | z8 | z8 | \
z8 | z8 | z8 | z8 | z8 | z8 | z8 | \


The chords posted above seem to fit in with this version very well.
Here's My Hearts's tonight in IRELAND, straight out of Andy's Irvine's songbook, "Aiming for the Heart":

X:1
T:My Heart's Tonight in Ireland
C:
L:1/8
Q:240
M:4/4
K:F
A2 A2 A3G | GA2A A2 G2 | F2 D2 D2 B,C | z6 F2 | FF2F3 D2 | C2 FF3 FG | G4 F2 G2 | \
G4 G4 | z6 CC | A3c A2 GA | z4 A2 G2 | F4 FE z2 | D6 DD | F3G F2 DC | z2 F2 F2 F2 | \
A4 G2 GF | z6 C2 | A3A A2 G2 | F3G A2 B2 | B3A F2 D2 | C6 AA | B3F D2 B,2 | CF4z FG | \
A3G z2 F2 | z2 FD D4 | z6 A2 | c3A c3d | A2 GA z3G | F2 F2 G2 DC | z6 CC | F3G F2 DC | \
z2 F2 F2 G2 | A4 G2 GF | z8 | \

There's nothing similar about them at all. So I doubt very much that he took one to make the other (unless there's another version of the Texas song, which is possible).

And John, here are the chords (again, out of Andy's book) in case you are still looking for them:

Verse:
| F | F | Bb | C |
|Bb F | F | C | C |
| C | F | Dm | Dm |
| Bb | F | F |Dm C | F |

Chorus:
| F | F | Bb | C |
| Bb | F |Dm C | Bb |
| Bb | Am | Am | Dm |
| C | F | F |Dm C | F |