The road from Dublin to Sligo is around 200 kilometres long and uses the M4 motorway for the first 60 km, and the N4 for the remainder which varies from a grade separated dual carriageway to a single. Starting from the N4/M50 interchange, upgraded from a be-trafficlighted roundabout to a hybrid stack/cloverleaf interchange in 2008, the road runs as a 3-lane dual carriageway past the private school King's Hospital and the Liffey Valley Shopping Centre, before bypassing the suburb of Lucan. The M50 to Lucan section was built on the cheap around mid-1984. The Lucan Bypass section, with a bit more money spent on it, was completed in 1988, severing the old village to the north from the newer areas in the south. The old route ran straight through the town's narrow streets. An old tramline once ran on these streets too, sadly removed many decades ago. Back on the new road, the Spa Hotel (named after a water spa) comes into view on the left just before the motorway section begins. The road runs very close to the Liffey at this point too, which passes just to the north.
In July 2009, a major project was completed which upgraded all of this route to 6 lanes, closed off most local accesses, and finally eliminated the cheap Adamstown/Newcastle Rd traffic light junction in the middle.
The old road ran onwards to the town of Leixlip, crossing the Liffey, ascending a steep hill, and crossing the Royal Canal, a very long canal which runs all the way from Dublin city centre to the oddly-titled River Inny in the midlands where it eventually ends up in Lough Ree. After the canal, it passed the sprawling Intel campus (largest in the world outside North America) and entered the town of Maynooth via a long, straight road. The town is famous for its university and lively student population. The next town was the sleepy village of Kilcock.
A 17.6 km motorway replacement for this whole route was opened in 1994. An extra intermediate junction 2A was addedin February 2003, primarily to serve major industries to the west of Leixlip such as Intel.
The N4/M4 route used to continue through the countryside and the small village of Enfield before reaching Kinnegad. At this town, the N4 continues northwest, while a new road is spawned, the N6, and heads southwest. The town had a name as a traffic blackspot as far back as 30 years ago, especially on bank holiday weekends when Dubliners would traditionally head down the country for a few days. The whole section from Kilcock to Kinnegad was replaced by a winding, tolled, 37 km westward extension of the M4 in December 2005.
Veering northwest after Kinnegad, the N4 continues as a dual carriageway as far as the large midlands town of Mullingar. The town was bypassed with good quality grade separated dual carriageway bypass in 1994, with the tie-in to the M4/M6 area added in July 2005.
Beyond this town, there are no further large towns. A series of villages such as the charmingly titled Edgeworthstown are encountered in the long, empty run up to Longford. It is eventually intended to replace the entire Mullingar-Longford stretch with a HQDC, though there is no timeframe for this.
The N4 then passes through the town of Longford, certainly a town that has suffered more than many from the effects of congestion and truck traffic thundering through its thoroughfares. Road surfaces in the town are perpetually falling apart, getting eroded shortly after repair by high traffic volumes. The town serves as the gore between the N4 heading up to Sligo, and the N5 heading west to Westport on the coast. A single-carriageway bypass was opened many years ago, but this only rerouted the N4, not the N5. It will take a fair amount of effort to sort out this town's unfortunate lack of investment given its crucial position on the country's road network.
The run up to Sligo is marked by a series of pleasant, small towns: Carrick-on-Shannon, the amusingly-titled Rooskey, the very hot Boyle, and the crazy-sounding Collooney. Many of these are due to be bypassed with low standard dual in the next few years - Rooskey's bypass opened in December 2007. From Collooney, grade separated dual opened in 1998 and an Inner Bypass opened in September 2005 completes the journey, ending up at the small junction at which it meets the N15 up to Donegal and the N16 to Northern Ireland and Enniskillen.
On 6th October 2010, the country's third MSA (Motorway Service Area) opened at Enfield, and on 10th June 2013, Junction 14, The Downs, opened to traffic along with the construction of a few frontage roads in the area.
There is a long-range plan to make further improvements to the N4 from the M50 to Leixlip, encompassing the Lucan bypass. These are described as a "reconfiguration" but no details are available. It is likely that changes will be made to the Liffey Valley junction (number 2) since this was built to a low standard around 1998.
Collooney / Castlebaldwin is under construction and making good progress.
Jct 5-7 Maynooth-Leixlip Widening, Mullingar - Longford - Rooskey and Carrick on Shannon to Dromod projects are in Feasibility Study.
The Collooney to Castlebaldwin scheme is flying along and may even come in before its target completion date of August 2021.
Mullingar - Longford - Rooskey is out for public consultation.
Here is a video of the latest progress on the Collooney-Castlebaldwin scheme.
Route options have been published for the N4 Mullingar to Rooskey upgrade. There is a virtual consultation website here and a brochure here.
The 14.7 km Collooney to Castlebaldwin dual carriageway has opened to traffic today (information from before the opening is here).
It replaces the last poor quality section of the Dublin to Sligo road.
It isn't the final scheme that will take place on that route, though, as Mullingar-Longford-Carrick-on-Shannon is planned to be replaced with more dual carriageway within a decade.
For now, the mainline of the new scheme is open while work continues on side roads and tie-ins.