The fact that 76 minutes passed between the first and second goals was due almost entirely to the visitors' goalkeeper Emma Higgins, who was outstanding, and her central defenders Ashley Hutton and Julie Nelson.
The hosts, for their part, turned in a performance which lacked the sparkle or panache of their earlier victories over the Faroe Islands and Bosnia-Herzegovina. There were subdued displays by many of the players while torrential driving rain in the first half didn't aid the spectacle.
"We'll start with the positives - it was a good three points, a clean sheet and two goals," said Signeul, who has been shortlisted for Fifa's women coach of the year. "Northern Ireland don't concede many and are hard to break down.
"In the first half we played too many long balls - they were successful against Bosnia, but because of the rain they bounced off the pitch tonight. The quality of the final pass was also lacking in the first half.
"The tempo got slower and slower and I told the players at half-time they had to speed it up. It was better in the second half but even then we complicated it too much.
"Of course we want to play like we did against Bosnia every time, but we're not that good yet. We still have a lot to learn."
A second-minute goal from Jane Ross hinted at another rout, but the Scots were a frustrated bunch by the time Jenny Beattie added the second. They had nearly all the play against a side who only very rarely threatened.
"We had 26 chances and probably should have scored many more, but the goalkeeper was very good," Signeul said. "That said, we're happy to have nine points after three games. We go into the Poland game on Thursday with confidence."
Scottish FA chief executive Stewart Regan made presentations before kick-off to three of the squad's longest serving players - Gemma Fay, Suzanne Grant and Jo Love - although only goalkeeper Fay, winning her 161st cap, started. The next celebration of the evening arrived soon after the game began.
Arsenal midfielder Kim Little latched on to a goal kick from Higgins, and struck a first-time ball behind the defence. Jane Ross nipped in to punish the keeper and put her side ahead.
In Scotland's previous two games, an early goal had been followed by other quick ones but this time the deluge came from above. The home supporters at the front of the main stand had to scurry to the back seats to avoid being further soaked by the mini-monsoon.
The conditions forced the home side to resort to more direct tactics, but although Jane Ross had shots saved by Higgins, the visitors' central defenders, Hutton and Nelson - the latter playing against several of her Glasgow City team-mates - coped comfortably with the numerous crosses and corners sent into the box.
Northern Ireland had their one worthwhile chance, a Sarah McFadden header which was just too high, before the interval, but with conditions easing in the second half, the home side found a rhythm again and redoubled their efforts to find the safety net of a second goal.
They continued to be thwarted by the excellent Doncaster Belles goalkeeper Higgins, who made several fine blocks and saves, before relief finally arrived in the 78th minute when Beattie fired in a low shot from 15 yards.
Earlier in the day, Group 4 favourites Sweden surprisingly just managed one goal in their away tie against Bosnia-Herzegovina. The Swedes required a first-half penalty from captain Lotta Schelin to secure the three points.
Scotland (4-2-3-1): Fay; Brown, Corsie, Beattie, Lauder; Lappin (Mulvey 70), Sneddon; L Ross (Love 81), Little, Evans (Murray 88); J Ross. Yellow card: L Ross 30.
Northern Ireland (4-2-3-1): Higgins; Hurst, Hutton, Nelson, Vance; Caldwell, McFadden; Doran, Shepherd (McDowell 72), Mackin; Furness (Callaghan 23). Yellow card: McFadden 38.
Referee: C Dorcioman (Rom). Att: 1580.