Yet, all it took for 'Belvo was one moment of magic. Three minutes from the end, Mongan was handed a chance on the left wing and beat two players to touch down, with player-coach Andy Dunne adding the conversion for a 10-9 win.
Summing up the match, 'Belvo's director of rugby Phil Werahiko said: "I thought Blackrock played extremely well, they played a lot of the rugby as opposed to ourselves. We couldn't control the ball, they did.
"But I suppose we have got a few players who can do one or two special things and that's what it came down to - we brought the ball forward and one of our players broke through a few tackles and that was it."
It had the makings of a famous victory for Blackrock which would have got them back on track after a bad run of form.
Centre Rob Keogh notched three penalties early in the first half to give his side the edge for most of the game, but 'Belvo can count themselves lucky that Keogh failed to convert a fourth penalty in the 67th minute to see the game off.
"We managed to fend 'Rock off but our discipline let us down at times," Werahiko told IrishRugby.ie.
"We were very fortunate that they didn't convert that fourth penalty (for 12-3), that was game shot and we wouldn't have come back from that.
"But I suppose that's the difference for teams at the top. I've coached against some very good sides in the past, particularly the likes of Shannon and Cork Con and even if they have an off day they'll produce something special at the end."
'Belvo have retaken the lead at the top of the Division 1A table, yet Werahiko is adamant that this means little with regards to the title hunt.
"I don't believe that being at the top at the moment means anything really. Last week we lost to Cork Con, who were top of the table, and really they are the best side and for them to lose today (against Young Munster), it doesn't feel like we're the top team at the moment, having lost to them twice.
"But the win does give us confidence. Every time you lose you have to come back and to beat a team like 'Rock today who were very committed was very difficult," he added.
Blackrock College boss Andy Wood could not hide his disappointment at the defeat, but offered deserved praise to his players for their hard work.
"I'm delighted with the effort and delighted with the way we played. We performed extremely well in that first half and we were certainly worth our lead at half-time.
"Again, in the second half we built phases and gave ourselves opportunities but the scoreboard certainly doesn't reflect the effort that we put in there."
The encouraging factor for Wood and his players is that the performance signals a massive turnaround of form from their last two outings against Dolphin and Shannon, which saw them suffer convincing defeats.
"We talked about how we were going to play this week compared to last week and we changed a few things and it ran for us," he told IrishRugby.ie.
"Certainly we were in a position to win that game and I'm bitterly disappointed, but by the same token we'll take it game by game."
Blackrock will hope that they can take this form into next Saturday's vital fixture against Garryowen. However, they will need to learn from the manner of their defeat at Anglesea Road.
"Our focus immediately shifts to next weekend and we have to go down to Dooradoyle and make sure we put in a bigger performance and get back on the winning trail," concluded Wood.
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