Ashley Vincent put the U's in front, only for Tope Obadeyi to secure a point for the in-form Robins.
Shirtliff was pleased with the performance and said: "I've got mixed emotions. I thought we played really well. I was disappointed with the goal we conceded, it came from a bit of hesitation and confusion.
"We did get a good counter-attacking goal ourselves.. It was a strange day losing our keeper David Lucas in the warm-up and we might have won it but we might have lost it."
Shirtliff added: "The final ball was a little too strong or the run wasn't correct but that sometimes happens."