Until 1993, all Dublin-US scheduled flight had to stop at SNN (same on the way back). There's a very powerful political lobby there and although the was a big push for direct flights, this was refused - partially because the minister then had her parliamentary constituency close to Shannon (a place called Galway) and would have lost her seat.
When EI told the gov't that if they didn't get the right to fly nonstop (which would have opened up many high yield markets), they'd be unable to replace the ageing 747-100s they had then and would thus have to give up US routes, they gov't relented and allowed a 50/50 split, whereby for every one direct flight, one had to go via Shannon. Obviously, this was supposed to be a short term transitory arrangement, but it has lasted since 1993 and even with EI being privatised next year, the gov't shows no sign of budging.
With regard to BWI, EI had already used up its quota of direct flights (the new route was started in Sept), so the flights had to operate via SNN. With any luck, some of the daily flights will be n/s.