The Cavan S/C coach Andre Quinn was with London Irish before joining Cavan last year. He's employed full time to look after development Squads and Senior team
A few counties have full time S/C coaches
Cavan had Peter Donnelly in that role before too.
I don't know a lot about rugby but I'd imagine London Irish have nowhere near the budget of Leinster rugby. Quinn is also a Down native I have found so it's likely he would liked to have relocated closer to home and enjoy a much less demanding cost of living, so it's quite possible he was happy to forego a fair chunk of his salary too.
None of these circumstances were in place when Dublin enticed Cullen over to them. Both jobs were Dublin based so he had no reason to forego a salary and it's worth noting that Leinster are one of the biggest rugby teams in Europe so it's surprising how Dublin can entice a guy to swap a similar role with a professional set up. Must be serious €€€€€ on offer from Dublin.