If you're talking about the 24th century, cloaking technology is quite advanced at this point, but there are still some limited defenses.
Romulan cloaking technology
- Not technically a part of the cloaking device, but Romulan warbirds use a quantum singularity to generate power instead of a matter-antimatter reaction that Federation and other warp drives use. Therefore, Romulan warbirds don't give off neutrino emissions as other cloaked ships do.
- Additionally, Romulan ships are equipped with nullifier cores, which balance the radiative emissions of the cloaked ship.
Weaknesses of Romulan cloaking technology
- While traveling at warp, cloaked ships radiate a slight subspace variance. The faster the ship travels, the more detectable this phenomenon becomes. Thus ships using Romulan cloaks generally limit their speed to warp 6.
- Additionally, when traveling at high warp, the competing power demands of the cloaking device and the warp nacelles causes incomplete cloaking as well as potentially permanent damage to the warp drive.
- Furthermore, while Romulan warbirds don't emit neutrinos, they do apparently register as high concentrations of tetryon particles. The closer the ship is, or the more ships are in a cloaked fleet, the more detectable these tetryon particles become.
During DS9: "Visionary", Jadzia Dax scanned the nearby space around the station out to a radius of 2 km. She didn't find anything at first, but then she began to focus on the lower subspace bandwidths and noticed traces of low level tetryon emissions. The emissions appeared to be orbiting the station in an elliptical path, and since there were no neutron stars nearby, the DS9 crew deduced that it must be coming from the quantum singularity of a cloaked Romulan warbird.
Later on, this same technique was used by DS9 to detect a large cloaked joint Romulan-Cardassian fleet entering the wormhole to attack the Dominion.
- The Romulan cloaking device also generates chroniton particles, which in theory could also be used for tracking and detection.
- A gravitic sensor net is deployed near the Romulan neutral zone, but next to nothing is known about them aside from their not being 100% effective. TNG 6x14 ("Face of the Enemy") also mentions the border being littered with subspace listening posts as a potential threat to a cloaked Romulan ship.
Other active-scanning techniques have been used effectively against Romulan cloaks, such as the Federation's tachyon detection grid. However, this detection method requires that the cloaked ship travel through (thus "tripping") the tachyon field generated by an active tachyon beam being transmitted between two ships or sensing stations. The single-ship implementation of this is the tachyon scan, which has a much more limited range.
Another known active-scanning technique uses antiproton beams. Romulan (and perhaps other) cloaking devices typically leave behind a trail of risidual antiprotons, which can be picked up using an antiproton beam. This is how the Jem'Hadar detect cloaked ships (including the Defiant)
However, advanced ships like the Scimitar emit neither tachyons nor antiprotons and are thus impervious to all tachyon- or antiproton-based detection techniques. Luckily, the Scimitar appears to be a one-of-a-kind prototype, though the Remans surely retained this technology and could possibly deploy it once again at some point (then again, the Klingons never did manage to mass produce Chang's unique cloaking device once the prototype was destroyed).
What would happen
The Star Trek: Star Charts show that the Federation-Romulan neutral zone is the most heavily defended Federation border in the 24th century. There are 44 Federation outposts/space stations (Romeo Outposts, Sierra Outposts, Quebec Outposts, Tango Outposts and Earth Outposts) lining the neutral zone, in addition to 6 star bases. To reach Earth, the Romulan fleet would need to pass through not just the gravitic sensor net, but it would have to get past tachyon scans by Federation patrols as well as Federation outposts, starbases, and the protected space of Federation member planets/systems:
- Argelius II
- Caleb IV
- Ophichiucus III
- Sirius IX
- Proxima Centauri
Any massed formation of dozens or more warbirds would easily be detected on long-range sensors by Federation border outposts even before they crossed the neutral zone and would be engaged long before they reached Earth. The Romulans would not just be waging war against Earth but against the Federation and all of its allies, most notably the Klingons. Between the Federation and the Klingons, the Romulans are effectively flanked from all sides, with only a 50 light-year gap that prevents them from being completely surrounded.
Strange readings along the neutral zone
Most likely what would happen is that Federation outposts would detect the large Romulan fleet massing on the other side of the neutral zone by their enormous tetryon signature. At this point, the element of surprise would be lost and the cloaks would only be good for avoiding the targeting scanners of intercepting vessels.
Starfleet would send its response fleet to intercept the Romulans and also deploy a tachyon detection grid if the Romulan fleet disperses into smaller 2-3 ship elements to try to evade detection. All Federation science stations and military bases would also be put on high alert, making tachyon sweeps for cloaked ships.
A war on two fronts
A communique would be sent to Qo'noS, as the Klingon High Council would surely want to launch its own offensive against their, now very vulnerable, sworn enemies. Like the Red Army during WW2, the Klingons would open up a new front on the opposite side of the Romulan Star Empire while Humans, Vulcans, Andorians, Xindi, Tellarites, Trill, Benzites, etc. (and possibly even Ferengi) joined together to attack the main Romulan fleet.
If any remnants of the Romulan attack force reaches Sector 001, they'd be engaged by starships assigned to local patrol as well as reinforcements from nearby sectors like Deneb, Denobula and Andoria. As the Romulan fleet approaches the Sol system, starships and attack fighters would be scrambled from Jupiter Station and the Mars colonies. Then the Romulan ships would need to penetrate the Mars Defense Perimeter sentry pods. And as the last line of defense, the Earth has the Global Security Net, which consistent of unknown planetary defenses but probably includes some orbital weapons and more attack fighters.
Either way, it would be more or less a one-way trip for every Romulan warbird involved and a suicidal move by the Romulan Empire. The Romulans only ever had probably less than 500 D'Deridex heavy cruisers at any time, and that number was reduced to less than 200 by the end of the Dominion War. Even at full strength, committing "hundreds" of warbirds to an incursion into Federation space would leave their empire completely defenseless against the Klingons and any other neighboring powers. Unwilling members of the Star Empire would soon rebel as well, further weakening the Romulans until the entire empire collapsed.
And it's extremely unlikely they would succeed where the Borg and Dominion have failed. So they would effectively achieve nothing and lose everything.