Chessical: MacDonnell plays a Modern/Robatsch defence, perhaps he believed that an "irregular" defence would particularly pose Mackenzie problems.
Unfortunately for MacDonnell, he failed to score in either game he used this defence in the match, and it did not go on to be a regular part of MacDonnell's tournament repertoire.
For Steinitz's take on the position as white see - Steinitz vs De Vere, 1866 (castle on
Q-side and advance the <g> and <h> pawns.
MacDonnell was correct in so far as Mackenzie missed the best way to exploit his greater amount of space (Bh6), but MacDonnell was also unable to fully exploit the dynamic potential of the set up.
The game is consequently level for most of its course, until <38...Kh7?!>. With <38...g5> a draw would have been a legitimate result. It would have allowed MacDonnell to block the attack on his <d6> pawn which his actual strategy fails to do. For example:
<38... g5> 39. Nd2 Qg6 40. Ne4 Rf4 41. b3 b5 =
<46... Rdf8?!> is an admission of defeat: <46... Rxf2 47. Kxf2 a6 would at least have made Mackenzie further work for his point.