There was very little between the two teams until McGee pounced in the 89th minute to meet Chris Kenny's pass and head in the winning goal. It was a day of double celebrations for Kenny as his partner gave birth to a baby boy earlier in the day.
Kenny was wearing the captain's armband for Declan Sheridan's side in what was a worthwhile test for the Defence Forces to aid their preparation for next month's European Championships.
The first chance of the game arrived on 20 minutes when Gerry Walsh roamed into a good position but was just off target with his header. The visitors then went close before Irish set-pieces began to cause problems.
After a scoreless first period, Sheridan made three changes and it took time for them to regain their rhythm until Jodie Dillon forced UK goalkeeper Matt Shortt into a fine save.
However, there was little that Shortt could do to deny McGee in the closing minutes as the home team secured the win.
Irish Defence Forces: Brown (Deegan h-t); Scullion (Fox 84), Long, L Delaney (Shortt 73), Grace, Dempsey (Duffy 79), Friel, Kenny, Poutch (McGee h-t), Gettins (Dillon h-t), Walsh (S Delaney 73).
UK Armed Forces: Shortt; Cushion, Evans, Dixon, Broadbent, Williams (Woodhouse 72), Stoneman, Kerr (Berry 74), Claisse, Todd, Prior.