I know there have been other threads over the years on this, some of them covering the full length of the canal and others the Dublin commuter belt. This is just some thoughts following a recent spin from Cross Guns bridge (Phibsboro) to Kilcock. The bike was my Van Nicholas Ventus with 25mm Conti GP4000 tyres, the conditions were good after a longish dry spell and my route is here: https://www.relive.cc/view/1012652493
From Cross Guns to Ashtown, the path is tarmacadamed but in varied condition. It ranges from about 3m wide at first to a narrow section near Broombridge that is a bit rough in places. Along by the Rathbourne/Pelletstown housing, it is fine but there are a number of barriers and kissing gates that are a pain.
From Ashtown to Castleknock/Blanchardstown it is great but a little beyond the railway station, you enter the Deep Sinking, probably the worst section of the whole canal. Not only is it narrow with a steep drop on one side but the surface is a mixture of clay, rocks and exposed tree roots. It's doable in dry conditions if you have good balance and bike skills but even walking it when it's wet is a pain. This section is a bit under 2km long and you emerge from it (hopefully) at Porterstown Road.
From here on, the path runs along the north bank of the canal and is a mix of blinded stone, short stretches of tarmacadam and longish sections of grass/clay. The blinded stone is fine to cycle on in the dry and it's possible to keep up quite a good speed, dogs and pedestrians permitting. The Tarmac is fine but limited to a few area near Leixlip and Maynooth and a great new stretch at Kilcock. The grassy sections are ok when dry if you can stay in the worn clay tracks. These are generally quite smooth but some are below the level of the grass and can catch you out if you touch the edges. Cycling on the grass is slower and quite bumpy.
I stopped this time a little beyond Kilcock as the next section to Enfield is nearly all grass. On a previous spin to Mullingar, this had not been cut for some time and I had to stop a few time to clear it from the derailleur. When cut, it can still be a bit bumpy although it's a nice section for a walk! In Kilcock I had a great scone and coffee in Timeless opposite the church where they told me they were getting a track pump for cyclists in need.
The way back from Maynooth on is one of my favourite routes into Dublin, staying away from main roads and entering the Phoenix Park at Farmleigh. The average speed obviously picked up but the canal route is quieter and more attractive. I rejoined the canal at Ashtown for the final leg back to Cross Guns.
Even in dry conditions, a bike with wider tyres and suspension would make the trip more enjoyable. In wet conditions, it would be almost essential. Notwithstanding this, it's well worth trying on a road bike and you can always bail out onto the roads or a train.